In a way, this is a trick question. I think most people just think of the string being released from the center as being essentially the same problem as the ball detaching from the end of the string. If the problem were the latter, the ball detaching from the string, the answer would indeed be "b." The reason it's "a" is because no string is an infinitely rigid body, thus of course it would take a non-instantaneous amount of time (I imagine no faster than the speed of sound in the material the string is made of) for the ball to experience a change in centripetal force coming from the other end of the string. A question arises, what's the maximum angle that the ball can continue to subtend after the string is released? I'm guessing it's equal to the length of the string in the ideal case (that is to say 1 radian) but have no idea what it would be with the best real world material.
I don’t think this qualifies as a trick question, but I do deliberately allow people to misinterpret the question. I love the question you posed and don’t have an answer. But one radian certainly seems reasonable. Interestingly, I believe the time for a slinky to collapse is essentially constant, regardless of how much it’s stretched. So this would suggest the same angle, even if a heavier ball was attached to the slinky. This, I’m guessing the angle that the slinky undergoes may very well be the maximum angle you are seeking. I may have to go watch the video again to see what this angle is!
@@AllThingsPhysicsRUclips My apologies, I didn't mean to imply that you were being tricky, just that it's quite easy to deceive ourselves with an imprecise base mental model. I loved this video. Thank you.
@@pataplan I took no offense. I agree that it's easy to deceive ourselves, and that's part of the point of this video. I ask a very specific question and most people will interpret the question differently than it's asked. We don't always hear things the way they are stated!
@@pataplan The constant angle is just for a specific setup. If you change the length of the slinky, or the mass of the ball, or the angular velocity of the table, then this angle will likely change. But it will remain the same as you move around the circle.
@@dejanraen8489 This is likely true for the same reason that the ball attached to the slinky "dragged' behind the rotating table. Air resistance would slow the ball down and cause it's trajectory to continuously keep changing. So if we're being very strict about the situation, answer b only occurs with inelastic strings and only in a vacuum.
@@jameshall5171 You, are correct! In the real world every thing is made of rubber ie. a spring. On paper "the math" predicts (b) because (b) is correct. Even if he didn't introduce the string into this hot mess, the attachment point to the ball has some spring to it. In other words a hardened ball barring setting on a hardened plate, gains a known flat spot and the plate a dimple where contacted. Just as the Voyager space probe, when slung around Jupiter, changes its velocity and trajectory Jupiter follows suit. Every force has an equal and opposite reaction! Full stop! Just like Newton observed. Newtons third law of motion stands true! Bozo went of on a tangent (get it) here (no pun intended) with scenarios' (a) and (c). It's like saying physics isn't a part of the universe. If we took the universe out of this equation...Well, there's no need in pondering that notion.
A good summary would be to say that releasing the string releasing the ball. The ball isn't released until the tension wave reaches it and therefore continues its circular motion.
@@JHBG1971 note dt the beginning he had a nine elastic string than he did test with a stretchable string. The slinky ideal is true to the elastic string but not so with a none elastic string. Also centrifugal force is true to the center off spin access. The extended arms in the center spin mechanism change the result.
This is why is in all physics exams I took the questions started with "Assume you have a system with friction-less couplings in a vacuum and a perfectly uniform spherical object connected by a rigid rod to an infinitesimally small single point" because once you have to take account of material tension, air resistance and even object widths then the question gets increasing more problematic to answer correctly.
@@labbeaj Because if we didn't assume, we'd know nothing. We could in principle improve our instruments near perfection, but even then, there lurks Heisenberg's Uncertainty Principle.
@@labbeaj Truth is different than fact. Truth is relative to the speaker, but facts are absolute. A belief is a truth. Truth may or may not be factual.
Yes! The whole trickery as I saw it was the difference in perspective - theory vs. reality. So, what if the ball was more massive compared to a very low mass, non-elastic string? This would be much closer to the theoretical perspective. Anyways, the exploration and explanation of the reality perspective was really great. Thanks!
My answer was (B) both when he asked the question and after he explained that it was (A) simply because I imagined the string being cut at the ball and not near the center... I would like to point out that I was only technically right by happenstance. Excellent video and a great thought experiment!
The moral for physics teachers is "say what you mean". If you just say ball on a string, we assume the simplified case of a point mass on an inelastic, massless chord. If your "correct answer" depends upon the mass and elasticity of the chord, you are obviously being deliberately misleading by omitting that information. The rest was well treated.
Surely not massless, but non-elastic. The ball keeps on moving in a circle because it does not "know" that the string has been released. With a non-elastic string this information should be propagated instantly, or at least at the speed of light.
That was my instant question when the problem was posed at the beginning of the video…. What is the mass of the string? Sure the ball continues in a circular motion, but only for a tiny tiny faction of a second. At the end of the day, I don’t like the initial question because it purposely leaves out key information to trick viewers into answering incorrectly.
The "trickiness" of this question means it is a riddle or puzzle or "brain teaser", not a plain "question". Sci-nerds too often ignore subtleties of language framing. It is important if you truly do not want to fool people who know a bit of physics. You _never_ want to deceive students, it is horrible teaching practice. So you want to warn them there is a riddle or puzzle, and the "obvious" answer from the usual case of the ball only getting detached has to be spelt out, so then it is a proper "brain teaser" not a trick. Any decent physics student will immediately think "conservation of angular momentum" and stands a fair chance of just intuiting _something like_ the right trajectory, which is satisfying for the curious mind, leaving a pleasant after-thought, not the distatse of feeling having been subtlety tricked.
Actually the straight trajectory tangent to the circular one is NOT an approssimation. Just as you showed with the movement of a falling slinky, you should only base your calculations on the center of mass of the system. In all of the real examples the center of mass left the circular trajectory in a straight line. If you were only considering the trajectory of the ball, you should release it without the string attached. This way the position of the center of mass will coincide with the geometric center of the sphere, therefore leading to the expected result of the ball continuing in a straight line.
You are exactly right. As I mentioned in my response to @jadegecko the center of mass of the slinky/ball system will move in a straight line once released, but it won't be tangent to the outer circle, it will be tangent to the circle of the trajectory of the center of mass. So the straight line trajectory of the ball tangent to the outer circle IS an approximation to what actually happens.
@@oo88oo Exactly. This isn't a "surprising result", it's a pedantic trick question. If you're trying to demonstrate physical laws about circular motion of "a body", you use a string because it's necessary to apply a force to "the body" for the demonstration. He uses that necessity to smuggle in a concealed fact about the "body" that we assume we're supposed to be considering. He made the "force" part of the "mass of the system". Congratulations. Newton's Laws of Motion haven't been broken and B is still correct.
Since we're not ignoring small details, the ball also has to rotate. We can explain it either as to conserve angular momentum since it will no longer be rotating, or because points in the ball have different speeds since they are at different distances from the center of the rotation.
The ball might also be subject to libration (pendulum action) in one plane or another. Libration would cease but the ball's rotation would be slightly different depending on just when the "pull" stopped. I think.
@vibratingstring Please note that I'm not out to start an argument. But I think that part of the point of this video is that "in the real world" every "system" has this "kind of delay". And that the length of the delay only differs in magnitude but never "goes away" completely. And the "shortest" delay You can get can be calculated by measuring the distance from the "origin of the holding force" to the "released(point)object", and dividing that distance by the speed of light. And I think this can(should) be said to apply even to "ideal systems", it's just that in most cases it's usually explicitly stated (in one way or another) that the "delay" and other real world factors that have "minor effects should be neglected" (e.g. air resistance, loads not being "points", etc.). But that doesn't mean that these "minor forces" don't exist in the real world, rather just that they can be ignored in strictly hypothetical cases. Best regards. t
Interesting perspective - I once did an experiment similar to this but I used a 9" nail with feathers on it like an arrow and spun it at high speed by hand at about ten feet of line. I had set up a knife so that when I wanted to "Release" the nail, I would drop down a little at the knees and let the string be cut by the knife near the nail - worked great and it really was traveling at high velocity and I believe that the tiny speck of fishing line left beyond the knife was so minuscule that it had very nearly ZERO effect on the "STRAIGHT" trajectory of the nail - Your ball that continues on the circular path is interesting looking but, in effect, is simply NOT yet truly released from the force holding in in the circular pattern. The sliding puck was a similar case because - although the puck lost enough friction to slide; it was still, partially, being restricted by friction.
That's very good clickbait. He claims that the ball is, shortly, continuing the circular pat. And yes, when the total system of the ball PLUS it's fixture are released AT THE CENTER POINT of the circular movement, the ball itself won't immediately move in a straight line. Fysics tells us, correctly, and repeatedly measured, that, when the ball, moving in circular motion (mark that the circular motion is measured in a system that is NOT rotating with the ball) So a ball moving on a circle, held on the circle by anything, and IS RELEASED FROM THE ANYTHING, the ball IMMEDIATELY stops changing its direction. That is: from the very picosecond when the ball is freed from its centripetal force, it continues moving but in a straight line, tangent to the circle in the point where the ball was released. In this vidéo, the ball stays linked to its SLINKY. It is not released from the 'slinky'. So that system leaving the circular motion is ball + fixture. So Release the slinky Slinky+ball have a centre of gravity, that before the release was moving on a circle, and suddenly, immediately, without any delay, starts moving in a straight line . I don't know WHO did that type of experiments. I only cannot imagine that some physicist and/or engineer didn't measure this. If David had let go of his sling, he would have hit Goliath with the leather line instead of the stone. But he released the stone. Somehow detaching the stone from the sling. At the right moment when the tangent was pointing to Goliath's head. Bingo. The stone, in a straight line, hit the giant and we read the happy ending of the story. If David had released the sling, not the stone, that (loaded) sling would have flown away. It's centre of mass going in the straight line. Leaving its circle (smaller than the circle where the ball was circling) on the known tangent. Why does he pretend that would be different for his contaption? Where in this video does he measure the path of the centre of mass of what is released? Maybe The standard problem is solved, so it isn't a problem any more. If the ball
Agreed that the issue is the 'friction' between the puck and the table. It allows an unbalanced Centrifugal / Centripetal forces because of the friction! The assumption that the 'friction' is equivalent to instantly releasing the puck from the tether to the center is completely false. Imagine this test done on an 'air table' with the puck circling a center point on the table with a rigid arm. Also, there is mechanism to release the puck instantly from the bar at any moment. Now, the puck is forced to move in a circle because of the bar. i.e. their are no unbalanced centrifugal / centripetal forces. What will happen when the puck is released? The centrifugal force ends instantly as does the centripetal force. What is left is the horizontal motion. It really will be straight line motion at a tangent to the circle.
And by removing the Sun he says earth 🌎 doesn't fly away for 8 minutes. Can information travel faster than the speed of light. I no then it's simply not true
Excellent video. It boils down to definition. When released from the centre, then you're no longer talking about a ball because the object is a [ball + string]. You'd have to release the ball at the radial end of the string to remove the string from the object, and then you will get answer B. It would have been good if you replaced the string with a metal rod with a release mechanism at the end to show the trajectory of the ball on its own.
Agreed, I think this would be more interesting. Presumably the direction of the ball would be tangential at the point of release, but would the ball have some spin and would the mechanism of release cause problems?
Or how about using a thread (or fishing line) and then take a very very sharp knife and insert from above into the plane of rotation near the ball. As you say with the slinky or the rubber tube the radial force on the ball is delayed by the tension propagation time (just like change in the gravity field on a planet if the sun vanished). The visual effect is interesting though. To say that people get the answer wrong is a bit disingenuous because the physics question is typically for an idealized world where the tension prop time is infinite. But I suppose the goal is to create a bit of drama.
The circular motion after release is because there is still tension for a small time even after release, use the release at circle, not at centre then result would be different
The question is more correctly posed as a linguistic one, rather than a physical sciences one. It boils down to how you define "release", because technically speaking the ball doesn't leave the [ball + string] system until the tension wave frees the ball - at which point it does precisely move tangentially to the point of "release". This is not a circular motion effect. It's a [ball + tensioned string]-system effect. Dropping a ball or spinning a ball are simply different ways of arranging the [ball+string] system.
I always cut my "massless" string at the ball. While I think your presentation of the question is disingenuous, I do love the video and how it sheds "light" on the finer details. Don't forget the mass (or more appropriately, the moment of inertia) of the string, this will have a similar effect without retardation (change your slinky to a solid rod). I stand by the answer "b", but admittedly to the question of motion after breaking/cutting the string at the ball. What I really like about this video is that it make one think about the real life impacts of the fictitious assumptions/simplifications we employ and quickly forget about.
Well, to me the term disingenuous has a rather negative connotation, as if I was trying to pull a fast one. And I wasn’t, really. Yes, I knew people would misinterpret the question, but the point is not whether (a), (b), or (c) is the right answer, the point is to watch the ball continue in circular motion after the string is released! Anyway, I’m happy to hear that you still appreciated the video despite my choice to let people misinterpret the question.
@@AllThingsPhysicsRUclips What if the "string" were completely inelastic, or rigid? Would the propagation be essentially instantaneous? I suppose the "string", being incompressible, would exert some additional effect, and perhaps how the result would play out would be influenced by the details of the release mechanism and the method of attachment of the "string" to the ball.
It was taught that way to me too, because it was meant to show what happens when the acceleration toward the center stops. If you leave the string, or slinky, attached it's a more complicated system. I don't think he was being disingenuous, just pointing out a problem with how the problem is presented by a lot of people. I've heard and read it presented both ways, my old physics teacher from high school presented it the correct way: what happens when the ball is disconnected, not the ball and string.
I'd just like say that the production on this video was outstanding. The clear writing, your eloquent narration, the beautiful graphics and slow motion edits to demonstrate your points. I think if you can pick the right topics you will have a million subs before too long.
@@AllThingsPhysicsRUclips Isn't the net force on the ball zero in both cases? Both when it is hanging vertically and when it is being rotated on the turntable the speed isn't changing. It's a constant radial distance from the center when in motion around the circle, and that doesn't change until the inner end of the spring reaches that radius and goes beyond it.
@@AllThingsPhysicsRUclips More slinky material! I came home with a box of them my junior year at Drexel. But we didn't have cameras in 1978. It must be so great in school today. This was great, thanks.
Thank you. I'm a retiree with a PhD in physics. You gave me something new to think about. I had a teacher when I was a student, Prof Brian Pippard (en.wikipedia.org/wiki/Brian_Pippard), who loved to demonstrate simple physical systems that gave unexpected results, such as spinning potatoes. Or how to use a glass of milk, a laser and a pencil to measure the astigmatism in your eye. I class this video as being in that mold.
As a young boy, I made and practiced throwing the bolas. I'm wondering if some of these same properties affected my throws? Had a lot of fun all the same.
@adrianstephens56 I don't see the misleading point about "circular motion" being in the same class as your experiments, since the latter demonstrate genuine and interesting effects, whereas the idea of this video is to mislead people with an abstract diagram -- signalling the usual abstracting away from nasty real-world effects, such as air-resistance, non-instant releases, wobbly centrers of rotations, etc. -- to essentially claim that the correct intuition most people have about circular motion is wrong. The intuition is not wrong, all this video does is to point out that the idealisation of an instant release propagation does not exist in the real world. That's a remark on the property of materials and (a point not made in this video) the limited communication speed between cause and effect, it is not a remark on "circular motion". I like your experiments.
@@coolcat23 Yes, but this is a physics video not a mathematics video so it's not so bad that it does that. I agree though, it doesn't sound as interesting as what the original poster mentioned imho.
@@coolcat23 I agree mostly but the propagation is very easy to overlook/miss. And I'd say in the real world, lacking this intuition could be a problem, as explained at the end of the video.
But only if 8 minutes is longer than the rest of your life, so there's that. But I was rather shocked when I learned that all forces, including gravity, only propagate at the speed of light. We didn't discuss that in a year of introductory physics in college (which is all I had). So while gravity is nonlocal, it isn't simultaneously nonlocal (I ran into this as an adult (layman) casually trying to understand how gravity could be nonlocal AND have particles as the mechanism to control the force.
More than that, I was thinking, what if the sun disappears, as explained for 8 mins, I will be oblivious as the earth keeps on rotating... but say, if the sun re-appears after 5 mins, what will happen? Will earth continue to rotate? There won't be any issues with earth's motion because of sun's disappearance for full 5mins?
@@varunkashyapv8383 Completely theoretical but probably: Earth would continue its orbit until it was no longer affected by the Suns gravity when it will move tangentially to its orbit in a straight line. By the time that the sun reappears, the Earth would have moved about 9000km which is not much compared to the fact the distance to the Sun varies between 147,100,000 and 152,100,000 kilometres however depending on where in its orbit the Earth was, it may just result in a very mild change to our orbit without noticeable effects to a substantial change to the min and max of the elliptical (either more elliptical or less)
As others have said. These kinds of questions assume idealised scenarios. Almost like the string vanishes from existence (like the sun in the final example). So B is the 'correct' answer. Great expansion of the concept. The falling slinky alone is a great challenge to the assumptions! Love this video
Ah, very thoughtful response. One principle of classical physics calculations is that one can use the coordinate system that gives the ease of calculation that is desired, and the result will be the same from other "correct" analyses in different frames when translated back to that frame. In this case, we also need to consider the assumptions of the problem as Darren notes. In physics class we would usually suggest the string was infinitesimal or insignificant mass. If not, then that needed to to be specified as part of the problem description. If assuming infinitesimal mass frictionless string, then the Cartesian frame is the easiest frame to use. Then what is meant by "release"? If instantaneously discontinuing the force or tension on the string, rather than letting the string slip slowly out of one's fingers, then very simple indeed and "b)" is clearly the answer because ball's motion is -Y direction and no forces to modify that. The drawings idealize over the actual motion of a person swinging the ball, because that itself would most likely be a non-circular motion because the person is not moving in a circular hand motion and all the interactions involved having stabilized. So the drawing belies the actual video example. If we are following the drawing, are we not using simplified assumptions like circular motion, so why not also infinitesimal string mass and instantaneous release? The video segment with the puck on a rotating disk is sort of a non-sequitur (though interesting). The point there is that after beginning to slip -- a matter of non-linear frictional forces that are reduced when the tendency to "stick" is overcome, are nevertheless forces imparted by the rotating disk upon which the puck is moving, which explain the partial spiral motion. Now a similar case might be rigid rod with significant mass replacing the string, but circular motion and instantaneous release. In this case the center of mass of the rod/ball system is useful, and also probably Cartesian frame. Then the rate of rotation will be constant at the moment of release, and the center of mass should follow path "b)" but of course not the ball but the CM. So a translating frame following the CM is useful, wherein the motion will be strictly circular, but then translate to the still frame to see the compound motions. Then extending what I said, the Slinky (tm) tends to demonstrate a transmission line effect (traveling wave). So at point of release, the change in forces are only apparent at the center part of the slinky system. In that manner the tension forces on more outer portions remain in effect until the traveling wave reaches that portion. Here a CM frame would again be useful, and would indeed the CM should follow the -Y path (as 'b') but the object is not rigid so the motions within that frame will include the traveling wave effect and be quite complex. Then in the end of course any real string is not only not massless, nor frictionless in air, but also has a modulus of elasticity so that it actually can be modeled along with its mass density as the same as the Slinky, but the wave travels must faster. Thus in the end, "a" is the answer for any physical string -- but the time frame before looking more like CM frame above would be very very short. AND of course -- the CM frame is not completely accurate because the air, presumably stationary to the reference frame, puts force on the ensemble and slows the overall frame and effects the parts.
@@gordonelliott7870I think repeating the puck-on-a-surface situation, but immediately removing the centripetal force by halting the turntable with a hard stop would come close to simulating the instantaneous release of a string. The puck will, for a short time, continue to move in the same direction it was going when the table was braked down to 0 very very quickly.
Good presentation and the slow motion video with the slinky spinning the ball to illustrate the effect of finite time required to propagate the information was very impressive. I was thinking of the "what if the sun disappears" case as an example but you mentioned it at the end. That being said, the question in the beginning was tricky in the sense that most people would assume you are using "fully rigid" spring. Could have started off by clearly stating that the string is elastic, or by not even posing this question but just saying that this video demonstrates the effect of elasticity or finite speed of propagation of information on circular motion. That alone in itself is incredible in itself, as you have demonstrated in the rest of the video. This tricky question at the beginning made it hard to take you seriously in the beginning, especially when it was immediately followed by the example of an object on a rotating turntable wherein the cause of the centripetal force is frictional force and the behavior you showed (it slipping and it following a curved path) was for a phenomenon not directly related to this topic, which you didn't even get into in the video! Could have avoided the sensationalism, just my opinion.
Very well demonstrated. I really appreciated the fact that you reiterated the importance of this by giving examples of how it could actually effect the outcome of systems.
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. The anti gravity is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
You have an incredible talent for making physics videos that are engaging for physics novices and experts! I can't wait to see what else you do with this channel. 😊 A note for physics teachers: standardized tests (AP Physics, MCATS, etc) will expect your students to give the straight line answer (B). This is typically because instead of asking about the string being released, they ask about the string being CUT, and the assumption is that it is cut at the location of the ball, so there is no delay while a wave propagates. If your students are comfortable with the material, be sure to point out this important difference.
Thank you for the kind words, and for the important disclaimer regarding tests. I think it's fair to say that ALL test questions on this topic assume that the force goes to zero instantaneously, for example, but cutting the string right next to the mass. It is definitely important that students be aware of the difference!
I think it is more subtle than that; really, in introductory physics we basically just assume that changes propagate instantaneously, rather than doing the messy business of taking propagation time into account. I think it is probably a good idea to point out that we are making this assumption, and how it simplifies what we are doing without substantially changing the physics involved. Ultimately, the question is trying to ask what happens to something in circular motion when it no longer feels a centripetal force. For the example of a ball on a string, the ball no longer feels the centripetal force only after the information has had time to propagate along the string, but that is close enough to being the same moment the string is released as to not matter in most instances.
@@AllThingsPhysicsRUclips ... The wave propagation speed in ideal strings is infinite because a) they are infinitely longitudinally rigid (Hook spring modulus k is infinite) and they have zero mass so the acceleration for any force applied is infinite which in turn leads to infinite speed.
There are a lot of comments complaining about the question being misleading, but I feel like those people are way too concerned about being right and how that affects their own egos. In reality, the video isn't about if you already know the answer, it's about learning something new, or thinking about things in a new way, and it does a fantastic job of it. It's very well communicated, a very solid length for this topic, and has a very good mix of theoretical and experimental sections. Great job and I'm looking forward to more videos!
It is truly interesting, that is for sure. I assumed 'C' wrongly, and now I'm wondering exactly where in their rotation a hammer thrower (sport) releases the hammer. Also how a stretched rubber sheet can be used to represent space-time; not with objects stretching it downward, but by objects stretching it inwards, towards themselves.
If the response to this video has been negative despite it being informative, then it is only because how the video phrased its central question. Essentially they are making it a trick question, to create a "gotcha!" moment. This is cheap and off-putting.
As an Electrical engineer, I only did physics in my first year of University (freshman year, but I don't live in the USA). Our physics, statics and dynamics problems always had disclaimers like a "light rope," "ignoring air resistance," "rigid bodies," etc. Therefore my answer was also B.
Where he gets you is his question , "what happen to the ball when I release "the string." and on this he's right, but as in his example of the 12.6 cm deviation that only equates to 1.26%, almost hardly worth mentioning unless you're slinging rockets to space via a rope or calculating the theoretical release point when David binged Goliath in the head and won the day.....lol. Mostly your right "b" is the right answer.
Your answer was correct. This is about semantics games, and the circular motion is about the SYSTEM, not the ball. You really can't DO elementary physics problems correctly without such disclaimers. I remember being VERY PISSED OFF during my first physics test, where my (excellent) physics teacher gave a problem that was very hard to think about given the reality of friction, in the context of a simple Sum of forces in two dimensions problem. After literally 40 hours over the weekend of working problems to be prepared, I almost confused myself and messed up the problem (he always gave unique problems unlike those in the book / lectures to ensure people knew how to THINK vs. memorize). I did calm down, and tell myself to trust the principles and techniques I had learned well and studied, and worked the problem correctly. Then, later in his office, we had a chat about clearly stating such assumptions, which he agreed with.
Another way to answer the question at the outset: At the moment the slinky is released, the system comprised of the ball and slinky COMBINED will have a center-of-mass trajectory that is straight. The motion of the ball plus slinky is not shown in the video, but it will be a tumbling motion around the uniformly moving center of mass. The system (ball plus slinky) is free of external forces (ignoring gravity) from the instant it's released. On the other hand, the ball feels a force from the slinky. The mass of the slinky is essential because it provides the inertia that allows the end of the slinky connected to the ball to maintain the tension that it had before it was released. That tension is of course the centripetal force that kept the ball on a circle before the release, and it indeed continues to force the ball onto a circle for some time. The tension at the outer end can change only if the deformation of the slinky slightly further inward changes, and this can in turn only happen if the tension further inward from that portion of the slinky changes, etc. The change in deformation at every location is an acceleration that happens with a delay dictated by the inertia of that portion of the slinky (Newton's Second Law). As always when inertia and tension of a medium compete, the outcome is a wave.
@@anyfriendofkevinbaconisafr177you are 100% wrong. Of course the slinky is analogous to the string, and the fact that you don't understand that, makes it obvious that you don't know all that much..
"The motion of the ball plus slinky is not shown in the video, but it will be a tumbling motion around the uniformly moving center of mass." You can see it very briefly at 7:52.
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. The anti gravity is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
Very nice!. As others have said it shows the importance of those words and phrases used to make elementary problems tractable: '...light, inelastic string...', '...a point mass...`, '...a rigid rod...`, `...rolls without slipping...` or whatever. Good to see videos like this that show how removing these kinds of assumptions has measurable and often surprising effects.
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. The anti gravity is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
@@vincecox8376Please either see a psychiatrist or a physicist - what a load of nonsensical bullshit - what exactly is special about center of the magnetic field, how exactly does magnetic field and physical vibration are supposed to interact in such a way to break laws of quantum mechanics that govern physical material properties of an object and why exactly granite, that which is an amalgam rock..?😂
@@MargoTheNerd Sorry your the one that needs help. I've been in the field of physics for over 50 years, I have many credentials , Please get yourself some mental health. I can put money where my moth is,,, I know exactly what I. talking about, BET YOU $10,000, on any of my points....
A cosmological analogy to this is that if the Sun was to suddenly poof out of existence, then the Earth would in-fact continue orbiting around the sun for about another 8 minutes. Essentially, in any physical system, there is a speed of causality of some sort - The information about "the ball was released" *always* takes some time to move from point A (the place of release) to point B (the ball). For the Earth rotating around the Sun, that speed is the speed of light - the ultimate speed of causality. This video is a cautionary tale to always make sure that the mathematics makes physical sense if you apply mathematics to physics.
Heh. I guess you didn't watch the video all the way through when you made this comment. I talk about this at the end of the video. Please watch again! 😊
Wave propagation is a fascinating thing, and is involved in surprising processes. It appears that it is involved in the swinging of a ball on a string (or slinky) in a circle...at least upon release. If you looked carefully, you could actually see a small reflected wave traveling back up the string when the tension wave reached the ball. The impedance of the ball is quite high compared to the string, resulting in the reflected wave. Same thing happens with sound waves, radio waves, and even water waves. Fascinating stuff.
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. This is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
@@vincecox8376You may want to correct your English to promote a fringe hypotheses. "Non" not none. "A lot" not allot. "Spread" not spred. "Huge" not hugh.
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. The anti gravity is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
Agreed, really fascinating. So you have the force propagated out as a slow moving signal, which doesn't update and tell the ball what to do in accordance with physics until it arrives, that's one picture. But then there is this second picture which is more local, about the center of mass and equilibrium and it just works out to match the first picture. Really mind blowing video to me.
Engineering with drag, elasticity, and mass, vs. Mathematical Physics, where to demonstrate gross effects of one activity most clearly, you can discount all other activities. So, definitely a trick question, when you pose it in simplified terms, then measure in the real world. Because if you had included, "on a string with mass and elasticity, through a medium with drag," only a few of us would even hazard a guess. So apart from the intellectual dishonesty, a great experiment to show the quite interesting real effect.
I watch a lot of science channels and this was such a unique video. I havent really watched anything on classical physics and forgot how interesting it can be.
That is mind blowing, you have to wait for the tension wave to reach the ball, because until then the centripetal force is still acting upon it... and obviously the less time it takes for the wave, the more answer b becomes a closer approximation of what happens. Great video!
I think it's a cool concept just a bit misleading with the initial multiple choice. The assumptions in the problem setup matter too much. B is only wrong if you assume the string isn't ideal, which I thought was implied. I took the question to be, "What happens to the ball once the effect of the string is no longer acting on it." Which would be B. But he really wanted to nitpick about the instant between the string being cut and the effect reaching the ball. On a practical level, the speed of sound is very fast in a string, so B is the best approximation of the path when not using a slinky string model. The slinky deliberately exaggerates the effect beyond what a typical string would exhibit. Assuming the string is ideal, and the equation for the speed of sound in a string being, sqrt(Tension/linear density), the speed approaches infinity as linear density approaches zero. Like he said, it's subtle, but it's important to be aware of the assumptions that are made in solving a problem. And when asking a question to leave as little to interpretation as possible.
@@RubixB0y and @allthingsphysics "I think it's a cool concept just a bit misleading" This Video does not show, that answer B is wrong. It shows (in an very impressive way!), that the speed of propagation of information (and thus causality) makes an enormous effect. In the moment, the information reaches the ball, it will move on the tangent. So answer B is correct. But when causality is transferred via the slinky, causality propagates very slow. As long, as the ball "does not know" that the other end of the spring was released, it will not change the trajectory.
@@amuller3101 I don't think it's misleading because I answer the exact question that's originally posed. Yes, I knew that many people would misinterpret the question, and I relied on that to make things interesting. But that's not misleading.
definitely a bit of a trick question. shows the difference between "releasing the string" versus "detaching the ball from the string". Under most circumstances the mass and momentum of the string would be considered negligible, but if its not then these two scenarios are completely different for the unintuitive reasons demonstrated in this video.
Hmmm...not sure I'd call it a "trick" question, given that I provided experimental evidence in three different systems, but no doubt I am focusing on a bit of a technicality. And technically, this doesn't really have anything to do with the mass/momentum of the string because the same thing would happen if there was a ball connected to the other end of the string. It is simply due to the finite travel time of the wave speed (which, admittedly, probably depends on the mass density of the string).
@@AllThingsPhysicsRUclips Sorry, I wasn't being precise in my language. In most simplified physics problem it would be the tension wave of the string not being considered, not the mass and momentum. That was how I should have worded it. And my first point still stands. This phenomenon would not be present if the ball were to be detached from the string/slinky at their connection point, rather than the string/slinky being released from the center of the circle with the ball still attached.
@@ThenameisMarsh You're absolutely right that this phenomenon would not be present if the string was cut right where the ball is connected. And I've seen the question posed exactly that way. But that's not very realistic; it's certainly not the way most people would actually do the experiment.
It's not a trick question. It is a challenge question meant to illustrate systems are complex. I first encountered this in college: the Earth does not travel in an exact elliptical path around the sun. The moon has mass, it and the Earth wobble during orbit. The center of the system is what does not wobble. @@AllThingsPhysicsRUclips
I would have liked to see a ball releised on the end of a string after the demonstration with the elastic, I get that it isn't really nessisary for understanding but would have "closed the loop" if you will in terms of explaining the topic. Very cool video.
Yes, unfortunately it would not show anything. The wave propagation at (approximately) 2,000 m/s, coupled with the lower frame rates we had to use because the camera was so far away, just made it impossible. I thought including that footage was simply not worth it.
@@AllThingsPhysicsRUclips But if you release the ball at the end, rather than release the tether, why would it continue on a curve path? Isn’t the curved path resulting from the tension equilibrium that then dissipates outwardly whereas by releasing from the ball end, the dissipation would occur centripetally and then you would see the ball take a tangential path?
@@AllThingsPhysicsRUclips move the camera closer and time the release. You get a short section but you might still be able to trace a curve (depending ofc)
The reason the ball seems to curve when it's attached by a Slinky after the Slinky is released is that the ball is in a neutraly balanced state and position. This is because it is actually being held in place by the resistance of the Slinky coils. Which is countering the centrifugal force equally, thus equilibrium. Therefore, C was the correct answer.
Got me with that one. When the connection between the ball and the string is released, the correct answer is (b)... But the experiments were great anyway!
The words "immediately on release of the string" do not mean the same as "when the ball is let go". The ball is still under the force of the tension of the string all the while until the propagated wave reaches the ball. It is at this point that the ball is "let go" and then it turns out that B was indeed the correct answer, at this point in time.
Interesting! I may have changed some things a little differently in your ball string experiment, but only bc you shared the set up and results of your tests! I appreciate all the thought, effort, and execution of the experiments! Well done!
The puck on the rotating table is governed by one other force, not mentioned in the early part of the demonstration. It is held down onto the table by friction. This changes the whole game because it can't possibly behave like a weight on a string. 5:16 In high school physics, we saw the same experiment performed where the rotating weight is an air puck on a steel table. Essentially frictionless. So when testing was quickly severed by a flame, the weight indeed left orbit in a tangential direction.
I wasn't crazy about the puck description, either. It seems to ignore the friction that still exists once the puck is moving (kinetic friction), and if you account for that the puck isn't a good analog for the ball on the string. I think the video would be better without it.
@@michaelporter1 And swinging a ball around his head produced and elliptical path, _not_ a circular one. False premise leads to false conclusion. Ah well; so much for post-truth physics, huh?
This is wonderful. I like that the "string" continues rotating about its center. And the connection to the gravity conundrum about the disappearance of the sun. Thank you
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. This is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
@@vincecox8376 Err ...... Who here is talking about antigravity, or ANYTHING you mention in this comment. Try to make comments about the material in the vid. No one is impressed
We were presented with this question by our university physics professor back in the early 90s. But in our scenario, the ball was released from the end of the string. So we determined that the ball would continue in an arced path between B and C, due to the centrifugal force and angular momentum.
And this is the truth of it. The premise of this demonstration was disguised in the introduction to the problem to mislead. It was obvious to me that your solution was correct, but it was not offered as a "solution". This allowed them to introduce the elasticity and reframe it as "briefly" and "subtle" during the discussion. Not impressed.
I don't think you can talk about centrifugal force in an inertial reference frame which I assume is the one we take when we try to explain this. Also I think angular momentum is perfectly conserved on a straight path as well so I don't understand your reasoning. If your answer were correct, what would provide a non-tangential force to change the direction of the movement of the center of mass? The only way I could see rotation playing any role is if there were some aerodynamic effects of lower pressure on one side ,thanks to the initial rotation, and airdrag
Really interesting video and presentation, Stil, I"m a bit puzzled by some things into the video. 1°) I have noticed that the tiny upper disc when you increase the speed of the downside sustaining rotating disc; not only at a certain speed start to glide radialy; but also spins on itself (seen on the larger disc but also after passing over the edge). 2°) When you think about the problem, you may see it: -As letting the rope and the weight go at a given time of the rotation; as such the tension into the string is proportional to the weight and to the number of rotations per second (rpm); then you may consider the string to be a spring elastically proportional to the tension; when releasing the rope, the weight continues its circular motion until the contraction wave (tension) from the rope hits it; then the weight and rope follow the radial trajectory. But this reaction is so fast that it must not be visibleon footage. --As a stone from a spinning sling, and this usually go straight radialy to its target when set free from the sling. Regards, PHZ (PHILOU Zrealone from the Science Madness forum)
Excellent video. I did not think of the string tension to have a material impact. Impact of this issue was really striking with the slinky. Thanks for making this video - learned something!
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. This is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
@@vincecox8376 What the heck has this to do with the material in this video? Really, you are all over the place with this same cut/paste comment Yadda yadda Kind of obsessive, methinx
Awesome! This video very much feels like the an early Veritasium or Steve Mould... perhaps Smarter Every Day-esc kind of content. In the future, you might want to put a small watermark in the corner of the video where the experiment is taking place (the automatic RUclips watermark is a bit small and does not contain the channel name) - that way most people that share the most interesting part (the actual experiment demonstration) will be helping to signal boost the channel :)
@BridbrainEngineer -- it pairs really well with the one Steve and Destin did together called "Which Way Will the Water Go?", about a spinning sprinkler, since it deals with the (non)intuitive nature of switching between rotating and non-rotating reference frames.
thank you @pataplan! All the experiments you've performed DO NOT duplicate the situation framed in the question, which is assumed, as noted by @pataplan, to be a detaching at the ball end, and so there is no elasticity effect, which could be though of as a centrifugal force holding the ball in a circular path. but interesting to ponder!
We’ll, in my defense, I did ask quite explicitly what happens to the object immediately after releasing the string. So the situation framed in the question is precisely what was investigated. 🙂
Excellent.I can see its important now to distinguish where the release happens- at your hand or if the string breaks at the point where the ball is connected.
Thank you so much for this video. I was pondering this exact question a few months ago and had also followed the logic of the slinky drop. My final question in fact was what would happen to earth’s trajectory if the sun was to suddenly cease to exist. Great demonstration of the physics of it all.
"What path does the object take immediately after releasing the string" is still "b", because it's the object that releases the string, AKA, a slingshot. What you are actually talking about is "What path does the object take immediately after the string is released".
How does an object release a string? What path does the object take immediately after the string is released [by the person swinging the object around!]
@@AllThingsPhysicsRUclips "how does the object release the string" - the object tears off, or release mechanism attaching object to the string is triggered. Slingshot.
Very stimulating, thanks. That is why in many statements of this problem is said that the string "disappears" or "disconnects at the ball side" (a relatively simple mechanism to do) so students focus on the main question. Even with a rigid bar this would happen just the sound velocity in the bar would be so fast that we could not see it (Ok there are come complications like the mass and inertia moment of the set ball-bar though). That is why some say physics is the art of ignore all the complications and focus on the essential (or like a professor I had used to say anytime you see a potential well approximate it by a parabola because we only want to solve for simple harmonic oscillator, we add complications later aka here there is more light). Imagine if Galileo in his experiences did not extrapolate the friction out: he would still be stuck in Aristotelian Physics. That is why we only tell that to students after they are mature enough to factor all the complexities.
We'd like to see a release mechanism on the ball end of the string. That would better match our early expectation of where you were going to take things.
The wave propagating along the string transmit the information that the string has been detached. As long as the information has not reached the ball, the ball must continue on the exact same path as if the string was still attached at the center. Congratulation on finding a medium with such a slow signal velocity.
At first I guessed B, then (as I expected) you said it was incorrect. After a little bit of thinking, I thought “oh is this something to do with the finite speed of the release of tension in the rope? Is it A? It’s A, isn’t it…” the issue is because of your lovely diagram I was imagining it in an ideal world where that lack of force transmitted instantly across the rope. But we don’t live in that world. Well played, you got me
Well, I hope you don't think I was trying to "get" you. On the other hand, few people would choose answer (a) and I think it's quite fascinating to see that answer (a) actually holds in a real-world setting!
The thing that leads people (even physicists) to go for answer B I guess is the unspoken assumption that "the string being released" means the centripetal force is immediately gone. We are so used to working with idealized models (infinitel stiff rods, massless strings) in which case that information is indeed instantaneously transmitted and the answer is in fact B. Once you challenge that assumption it quickly becomes clear that the answer is A in a real world setting. I fell into the trap of saying B myself, though I knew that was gonna be wrong, because why else would you make a video about it. Quickly realized where you were going with it when you started talking about the slinky. Still though, you're right about how striking it is to actually see the effect! It looks so damn strange, even though you intellectually know that's how the motion must be. Terrific video!
Yes, we (physicists) often simplify things to get across the basics, which is obviously the way you need to teach the subject, but once you've got the basics I find it fun to think a little more deeply about things we often take for granted.
In the question posed as a test, we were given imprecise information and told to use it as if it were perfect. A ball swung on a string by a person is not going to describe a perfect circle, but we are expected to assume that it does act that way. Similarly, the wave traveling down the string is estimated to travel at the speed of a rifle bullet ---obviously far too fast for us to be able to measure and use in the problem. So just as we are expected to assume that the ball is describing a perfect circle, it is only reasonable to expect that we will assume the instantaneous release of the ball. So I suggest that for the purposes of this test, answer B is the correct answer. It's really dishonest to add a lot of other conditions to such a test after the fact, in my view.
This is wonderful! I have taught physics for a long time, and this isa. very accessible extension of the core physics of circular motion. Thank you for presenting it so well.
Really awesome video, 12:20 I was struck with the realization of "phonon!" and wave propagations etc and it's really cool you could show that. I'm a materials science undergrad (senior)
Glad you liked the video! I hope you'll consider subscribing and sharing the video with others. If you're a materials science major, I've got a couple of other videos coming up that I think you're going to love!
I was under the impression that you were referring to releasing the string at the ball end. In this experiment the rotating object (ball) isn't being released, it's a ball and attached line. In any of these examples, forces acting on the ball causing it to change its direction are present momentarily. In the experiment where the puck is on the rotation table, the friction acting on the puck isn't immediately released either, as the puck is still in contact with the rotating table and only moves once it has overcome the static friction. Can we see the experiment again with the ball releasing at the ball end?
Thank you David for this surprising result. The explanation became complex because of the slinky wave, the elasticity of the bungee, and the extensibility of the string. The experiment we performed for a science fair 60 odd years ago in Brighton, UK was much simpler: a lead fishing weight on some monofilament. The monofilament was designed to fail at the weight end as the rotational speed was increased. We only had cine film to record the event. Of course, we concluded that the path was indeed tangential because that is what theory told us would happen. Within experimental error, that is! Seriously, with a high speed camera and the fracturing monofilament, you would find the expected result BUT your video would have been a lot less interesting and thought provoking!
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. This is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
It is neat to see the ball remain in place for a moment as a stretched Slinky drops. It from the gravity force downward is balanced with the tension of the coils trying to collapse upward. So net zero movement. I wonder what the record is for making a ball remain motionless in the air? Cool demonstration.
I think its improtant to clarify what "relaasing" the string means. If it points to the instant when the centripetal force disappears/weakens, then B would be correct, right? In effect the question you posed is equivalent to "what would happen if the Sun disappeared". We would continue to orbit for another 8 minutes (im disregarding the difficulty to define an agreeable point in time for both Earth and Sun in which the Sun should disappear). The only difference is the wave is now gravitational and much faster.
the ball would always move in circular motion for a brief moment in time even theoretically as the tension wave even in a massless string can not travel faster than the speed of light ergo the ball is very briefly in circular motion both in theory and practice
I love the combination of esoteric/flat-earth aesthetics and actual, real science being done. The contrast couldn't be larger. Really cool! I'm guessing if the release happens on the ball-side of the string instead of the slinger-side of the string the straight tangential line would be accurate. My gut feeling says if the ball was released with the slinky still attached, not only would it follow trajectory a as demonstrated in the video, but after the wave of the spring has released the tension that kept it on the circular trajectory, (again gut feeling) it should probably then actually start curving outward, basically again circular motion but away from the original circle. There is inertia in the string/slinky and that has to go somewhere 😅
17:49 Seeing your ending graphic with the *solar eclipse* just now is somewhat poignant and poetic, since I got to see totality of the _April 2024 solar eclipse_ from my back yard just a couple of weeks ago.
I saw the 2017 solar eclipse in totality in Salem Oregon and it was spectacular! That was the motivation behind my banner and logo. Glad you got to see the recent one. (I was in Killeen, TX this year and we lucked out with the clouds…not quite as spectacular as in 2017, but still quite awesome!)
Of course, if we want to be sufficiently pedantic, the ball can't swing in a perfect circle to begin with because of subtle distortions in spacetime. Physics is always a question of which effects are small enough to be ignored. Leaving an ever more accurate and pedantic series of "but actually's" as you take into account the heisingberg uncertainty principle on the balls position and the effect of the gravitational waves produced by the strings release on the ball.
Certainly an interesting situation, yet I agree with @pataplan the proposition was a little misleading or ambiguous. I too assumed the release point was at the outer end of the string, in which case the answer would be "b". Your results were surprising, yet after thinking about it, it does make sense but only because of the elastic tension in the tether, as it continues to provide a force on the ball until it is completely dissipated. I would like to see what would happen if the tether was rigid, say a carbon fiber rod or something with almost no tensile elasticity, release it from the hub as here, and then see what happens. I believe the ball then would continue in a still curved path, but no longer matching the radius of its former circle. In this case I imagine the center of mass of the ball/carbon fiber tether combo would instantly begin moving in a tangential trajectory, however the ball and tether would now begin rotating along this straight tangential trajectory, around the center of mass of the combined unit, and thus the ball itself would oscillate in its path around this center of mass. First video of yours I've seen, instantly subscribed and shall have to see what else you explore. This is the sort of stuff I'd stay in the classroom in high school during lunch time to argue/discuss with the other nerds! Love it!
Great video -- you learn something new every day! I'm assuming that if the release point was located where the ball is attached to the string (instead of near the centre of the circular path), then we'd see the ball immediately follow the tangential direction. However, I'm curious about whether the ball would experience the Magnus effect at all soon after, which could curve its trajectory sideways. Since the ball is already experiencing some rotation while its still tethered (1 full rotation per revolution, similar to the coin rotation paradox), I guess I'm wondering if that rotation is preserved after the ball is released? Would it follow the same principle that you demonstrated in this video? Thanks!
Well, I'll be honest, I hadn't thought about that. But you are correct that the ball is rotating and I believe this rotation would continue. Thus, if we don't neglect air resistance then the Magnus force would certainly be acting, though the rotation rate is pretty small so this force would likewise be quite small.
Yes a release at the ball would more closely approximate the tangent as not only would it more closely approximate the ridged body assumptions of these algorithms, it wouldn't have the string in the ensemble, which needs to be considered. While the Slinky's bottom doesn't move, the total mass of the spring is being accelerated as one would expect under gravity. IMHO some of this is a product of the limitations of the Newtonian approximations, Euler angles, and the cross product. In the GR case of the sun disappearing, the Earth is mostly following a geodesic, or the straight line on a curved surface. There would not be any perceived changes in angular momentum at all from the earth's reference frame. Just what is a straight line would change (ignoring tidal and other smaller effects) It is useful to replace fictitious force with apparent force when talking about effects like centrifugal force in Newtonian mechanics and for the apparent force of gravity under GR. These apparent forces are an artifact of the chosen frame of reference. Choosing the ball's perspective when attached to a string, and not the string ball assembly as a whole is the cause of the apparent paradox here. Spherical cows (oversimplified models) allow for practical models with simpler math. Euler angles, the cross product, and the fact that SO(3) is topologically a 3-torus really move your above question into the Grassman algebra realm for intuition IMHO
Really loved the way you built up from slinky to string! Would be interesting to see the path traced by the COG of the entire released system, which may actually continue on a tangent from it's position at the time of release.. Thanks!
That's not correct, again. 🙄 the COM travels rapidly up the string towards the ball after release. But since the string is still attached to the ball, we already know what happens to the ball+string system, as it's what this video is about. But if you are to now claim that the COM immediately moves tangentially from the circle at the moment of release, then your whole premise of this video is incorrect! If a much-more-massive string is used, the COG never reaches the ball, but stays along the string somewhere. Therefore if the ball+string COG system as a whole moves tangentially immediately, the net effect on the ball is **also** to move away from the circle. This would be very clear with a massive string. With a low-mass string it just **appears** that the ball doesn't stray from its circular path until the tension wave releases it from its tether; but actually it's merely that the COG change relative to the tension wave is not humanly perceptible. It has after already moved off the circumference. Using a heavier string would demonstrate this clearly.
@@nowandrew4442 I think you are mistaken. Consider the ball and string as the system. As soon as the string is released, then there are no forces acting on the ball/string system. Hence, the CM of the ball string system will move in a straight line with a constant velocity (assuming no air resistance). This line will be tangent to the circle that the ball/string CM was originally moving along, not tangent to the ball's trajectory. The ball continues to move along the circle because the string is moving toward the ball. These two motions are what keep the CM of the ball/string system moving in a straight. line.
@@nowandrew4442 that's a great way to look at it, yes. It's not so much the mass of the string but the rigidity of it that is the core of the unintuitive motion which makes the experimental result so interesting though. (edit - the question was where does the *ball* go, not the COM) Other than that bit you seem to agree that a rigid system if viewed as a discrete point (hence at the COM) would continue straight on once released? This is what makes engineering the fun branch of science, sometimes life sneaks in a bit of "elasticity" to keep our minds engaged!
@ creator But then A) would be incorrect. If the COG moves **immediately** tangentially, then the tension is not the key actor. This is not the case. There is a necessary interplay between the tension wave and the COG. It can't be perfectly tangential at the moment of release. The COG *also* moves tangentially **only once the tension wave passes it** - not immediately after release. Because the COG is tethered just like the ball is. We don't see the effect of the COG release because the COG is too close to the ball, when it's light-string+ball. Use a heavier string though, and we would see the ball's movement not be tangential as the COG is already dragging it in its (the COG's) tangential path after tension wave propagation. So the true story is: it is not the ball that awaits tension propagation, but the COG. When the tension wave reaches the COG, the COG moves tangentially. If the ball is sufficiently massive compared to the string, the COG is in the ball and those movements appear to be the same. But they are not.
Great video. This is about the level of physics I can follow (though not without some effort.) Once I accepted the premise that Person A at Point B is wearing a tie-dyed T-shirt in 2023, it was fairly easy.
Glad you touched on the sun disappearing thing at the very end, because that little fun fact about GR was the first thing that came to mind when I saw the first slinky test with the turntable. 😊
This is why I love practicing with a sling or bolas weapon. Takes a lot of skill to time your release with the swing if you want the stone to hit accurately. Also very similar to knife throwing, except knife throwing has the additional element of the knife itself wanting to spin, so to ensure the blade will hit the target, you have to actually stroke the handle of the knife as it releases to offset the knife's rotation as it travels along its arc to the target. That particular method is often called the "no spin," or "thorn throw" technique. Takes a lot of practice to not only throw accurately, but to ensure the point of the knife makes contact as perpendicular to the surface of your target as possible. Example of this can be seen here: ruclips.net/video/oqu1hm9RKr8/видео.htmlsi=Io-K2eekpQlBUmc1
Is it not true that the slinky and rubber band are still exerting force on the ball, PULLING theball toward the center, until tension is equalized, which keeps the ball from leaving the trajectory, until all force toward the center is gone; THEN the ball continues forth as tho there were never an elastic force involved?
Sorry, but this video is a bit misleading. He considers the elastic property of the string but not the mass. I like that he identifies the center of mass. He mentions that it is air friction that causes the slinky or ball not to be perpendicular to the tangent of the circular path the ball travels at the ball. I assure you that it is not the only (and probably not the most) cause for that phenomenon.
coriolis force applies where the orbiting object is in contact with the rotating reference frame. if a ball is attached to a string ans the string breaks, there is no such contact and the object is released tangentially. This is how a sling shot works. A satellite realease system has also been devised on this principle. The “rotating disc with sliding puck” system is different because, although the puck is slipping, there is still a frictional force, because the puck is still in contact with the disk.
Vrlo kvalitetno objašnjeno.Ja sam nešto slično pokušavao pokazati sa sekirom ali nisam imao dovoljno kvalitetnu kameru da to i dokažem.Pa su me ismijavali. Ja sam naime tvrdio da ima tu nešto više od običnog udaranja i da se maksimalni efekt postiže jednom petljom a ne pravocrtnim kretanjem sekire prema drvetu.Hvala za ovaj kvalitetni video.Pozdrav iz Hrvatske.
I disagree. If you just released the ball from the string it would follow the tangent line. Since you released the ball with the string/slinky or whatever held it in tension the mass of that object and its spring forces change the motion of the ball.
Well you sure did. I was thinking what the ball would do independently from the string the whole time but I should have gone back and listened to the question more closely.
@@wesleyashley99 Well don't feel bad, a huge number of people misinterpret the question, which is one of the reasons I think this is a fun video. They hear something different from what's being asked, and then typically yell at me and say that I'm not being fair! But I'm just making use of some well-known psychology to lure people in so I can show them something fascinating! 😉
Very interesting ! A year later I watched again and now I feel it's like the "Object in question" can be a ball or particle in which B is correct or if the "Object in question" was a three foot long ball made of rubber or a slinky which introduces many more factors that take us away from the basic principal that "Inertia wants to travel in a straight line" AWESOME THOUGHT EXPERIMENT THOUGH ! ! ! THANK YOU , it was so good I had to come back a year later to watch again ! ! !
This was an amazing demonstration, thank you! What I loved the most is how it builds intuitions about the different reference frames involved in this system to create a full understanding of their combined effect. Beautiful!
I was sorta following along for most of the explaination on a "this doesnt seem right but i trust you cause you sound like you know what youre talking about" Basis, but with the gravity example at the very end it finally clicked as i have spent more time thinking about that sort of thing and suddenly the whole phenomenon seems intuitive!
Thanks so much for the comment! I'm glad you enjoyed it. There will be many more videos, I promise, but it's difficult to find the time when you have a day job.
I dunno, the behaviour of the slinky & the ball on the slinky is exactly what I expected and seems totally normal to me. After all it's a slinky, not a rigid metal rod. Is that weird?
Great video. Unless I'm wrong if the release is right at the ball the tension wave delay would not happen and the ball would immediately travel on a tangent line to the circle. Correct? So answering the question correctly would depend on knowing where is the release occuring. Correct?
Reviewing he did say, "immediately after releasing the string" not immediately after releasing the ball. Still, it would be more precise to word the question such as "immediately after releasing the string at the center of rotation."
To be fair, the problem was stated as, YOU are swinging a ball around on a string (with a video showing someone doing so). How else would you release the string except from the center? 😉
Given that the ball is not being swung by an infinitely rigid rod/object, it will take a “while” for the ball to realize that the string has been let go of.. so it does follow that the instantaneous direction of the motion of the ball right after the string is cut/released will still be along the circle… fascinating video, lovely to see the slinky experiment
I'm doing a course on nonlinear dynamical systems. It comes at this kind of question in a different kind of way, but it's cool that I had the right intuition. I would have said (b) a month ago
What would happen to the ball's trajectory when it is held by an electro magnet to eliminate the 'slinky-effect'? My guess would be that the direction will be what classic mechanics predicts. Answer B would then be correct 'again'.
I have a question related to the ball hanging lagging behind the attachpoint of the spinning wheel. You suggested this was because of friction. But couldn't it be that the same principle that makes the ball keep spinning when let go, causes the ball to spin behind when it's accelerates in a circular motion?
Not quite. When the ball is moving around in uniform circular motion, the only forces acting on the ball are gravity, the tension force from the slinky, and air drag (there's nothing else touching the ball). Thus, air drag is the only force that could be responsible for this effect.
@@AllThingsPhysicsRUclips Thanks for your reply, I've been thinking about it and its been quite a while since my last mechanic's class. I feel like there is more going on for explaining the behaviour of the ball. I understand the ball needs some acceleration to maintain a constant speed and it has to obtain this from the strings, so an angle is necessary. But does this explain why the ball is lagging less when the noodle is used instead of the slinky? I understand more friction with the springy, but this doesn't explain for me the amount of lag. I think it has something to do with the tension of the string. If the string wouldnt lack behind the radius would be bigger and so a stronger centripetal force would be needed. So the string needs to be longer, The only way for the stringy to be able to be in equilibrium with the centripetal force and the radius is to start lagging behind. I have to outthink this more, but I hope you follow my reasoning. What do you think?
@@boloboyz7835 Actually, the angle is not necessary to maintain a constant speed. Once the ball is in circular motion, if there were no air drag there would be no lag angle. The reason the lag angle is so noticeable for the slinky is because the slinky is very light, and so the ball is also very light. These light objects are strongly affected by air drag. However, it take a heavier ball to stretch the noodle to an appreciable amount, and the heavier ball is much less affected by air drag, so the lag angle is much smaller. Does that help?
id like to ask about the time from when the pivot point end of the slinky ( or that of the elastic cord ) reaches the ball .. then the ball travels in a straight line, like in choice B, as can be seen in the slomos .. id like to propose a different experiment, but i think yours has more practical uses .. same setup, in general, but there's a provision for ball release at the far end of the slinky and the string. i think that this would have the theoretical result that physics teachers usually discuss in class. thanks for the video lecture, it's very enlightening. as a math teacher, i have always been in love with tangents and their real world applications, and your video is a great revelation.
In a way, this is a trick question. I think most people just think of the string being released from the center as being essentially the same problem as the ball detaching from the end of the string. If the problem were the latter, the ball detaching from the string, the answer would indeed be "b." The reason it's "a" is because no string is an infinitely rigid body, thus of course it would take a non-instantaneous amount of time (I imagine no faster than the speed of sound in the material the string is made of) for the ball to experience a change in centripetal force coming from the other end of the string. A question arises, what's the maximum angle that the ball can continue to subtend after the string is released? I'm guessing it's equal to the length of the string in the ideal case (that is to say 1 radian) but have no idea what it would be with the best real world material.
I don’t think this qualifies as a trick question, but I do deliberately allow people to misinterpret the question. I love the question you posed and don’t have an answer. But one radian certainly seems reasonable. Interestingly, I believe the time for a slinky to collapse is essentially constant, regardless of how much it’s stretched. So this would suggest the same angle, even if a heavier ball was attached to the slinky. This, I’m guessing the angle that the slinky undergoes may very well be the maximum angle you are seeking. I may have to go watch the video again to see what this angle is!
@@AllThingsPhysicsRUclips My apologies, I didn't mean to imply that you were being tricky, just that it's quite easy to deceive ourselves with an imprecise base mental model. I loved this video. Thank you.
@@AllThingsPhysicsRUclips also, you surmise contstant angle irrespective of weight, which seems reasonable, but what of higher angular velocities?
@@pataplan I took no offense. I agree that it's easy to deceive ourselves, and that's part of the point of this video. I ask a very specific question and most people will interpret the question differently than it's asked. We don't always hear things the way they are stated!
@@pataplan The constant angle is just for a specific setup. If you change the length of the slinky, or the mass of the ball, or the angular velocity of the table, then this angle will likely change. But it will remain the same as you move around the circle.
and this is why all those Applied Math questions always stated " a non-elastic string" where they would assume the reaction to be instantaneous.
wouldn't the ball in reality go in direction somewhere between b and c actually?
@@dejanraen8489 This is likely true for the same reason that the ball attached to the slinky "dragged' behind the rotating table. Air resistance would slow the ball down and cause it's trajectory to continuously keep changing. So if we're being very strict about the situation, answer b only occurs with inelastic strings and only in a vacuum.
Classic physics assumptions lol. "Suppose a perfectly spherical mass attached to a perfectly rigid string in a perfect vacuum..."
@@jameshall5171 You, are correct! In the real world every thing is made of rubber ie. a spring. On paper "the math" predicts (b) because (b) is correct. Even if he didn't introduce the string into this hot mess, the attachment point to the ball has some spring to it. In other words a hardened ball barring setting on a hardened plate, gains a known flat spot and the plate a dimple where contacted. Just as the Voyager space probe, when slung around Jupiter, changes its velocity and trajectory Jupiter follows suit. Every force has an equal and opposite reaction! Full stop! Just like Newton observed. Newtons third law of motion stands true! Bozo went of on a tangent (get it) here (no pun intended) with scenarios' (a) and (c). It's like saying physics isn't a part of the universe. If we took the universe out of this equation...Well, there's no need in pondering that notion.
A good summary would be to say that releasing the string releasing the ball. The ball isn't released until the tension wave reaches it and therefore continues its circular motion.
That’s it in a nutshell!
@@JHBG1971 note dt the beginning he had a nine elastic string than he did test with a stretchable string. The slinky ideal is true to the elastic string but not so with a none elastic string. Also centrifugal force is true to the center off spin access. The extended arms in the center spin mechanism change the result.
This is why is in all physics exams I took the questions started with "Assume you have a system with friction-less couplings in a vacuum and a perfectly uniform spherical object connected by a rigid rod to an infinitesimally small single point" because once you have to take account of material tension, air resistance and even object widths then the question gets increasing more problematic to answer correctly.
LOL! Yes, you do need to be a bit careful!
Assume...
Why do we need to assume?
Assumptions are almost like beliefs. Neither are true.
Beliefs are not true? Well, they are not false either. Believing or not has no bearing on whether something is true.
@@labbeaj Because if we didn't assume, we'd know nothing. We could in principle improve our instruments near perfection, but even then, there lurks Heisenberg's Uncertainty Principle.
@@labbeaj Truth is different than fact. Truth is relative to the speaker, but facts are absolute. A belief is a truth. Truth may or may not be factual.
The moral for physics teachers is “don’t forget to specify a massless string”😁
Yes! The whole trickery as I saw it was the difference in perspective - theory vs. reality. So, what if the ball was more massive compared to a very low mass, non-elastic string? This would be much closer to the theoretical perspective. Anyways, the exploration and explanation of the reality perspective was really great. Thanks!
My answer was (B) both when he asked the question and after he explained that it was (A) simply because I imagined the string being cut at the ball and not near the center... I would like to point out that I was only technically right by happenstance. Excellent video and a great thought experiment!
The moral for physics teachers is "say what you mean". If you just say ball on a string, we assume the simplified case of a point mass on an inelastic, massless chord. If your "correct answer" depends upon the mass and elasticity of the chord, you are obviously being deliberately misleading by omitting that information. The rest was well treated.
Surely not massless, but non-elastic. The ball keeps on moving in a circle because it does not "know" that the string has been released. With a non-elastic string this information should be propagated instantly, or at least at the speed of light.
That was my instant question when the problem was posed at the beginning of the video…. What is the mass of the string? Sure the ball continues in a circular motion, but only for a tiny tiny faction of a second. At the end of the day, I don’t like the initial question because it purposely leaves out key information to trick viewers into answering incorrectly.
The "trickiness" of this question means it is a riddle or puzzle or "brain teaser", not a plain "question". Sci-nerds too often ignore subtleties of language framing. It is important if you truly do not want to fool people who know a bit of physics. You _never_ want to deceive students, it is horrible teaching practice. So you want to warn them there is a riddle or puzzle, and the "obvious" answer from the usual case of the ball only getting detached has to be spelt out, so then it is a proper "brain teaser" not a trick. Any decent physics student will immediately think "conservation of angular momentum" and stands a fair chance of just intuiting _something like_ the right trajectory, which is satisfying for the curious mind, leaving a pleasant after-thought, not the distatse of feeling having been subtlety tricked.
Actually the straight trajectory tangent to the circular one is NOT an approssimation.
Just as you showed with the movement of a falling slinky, you should only base your calculations on the center of mass of the system.
In all of the real examples the center of mass left the circular trajectory in a straight line.
If you were only considering the trajectory of the ball, you should release it without the string attached. This way the position of the center of mass will coincide with the geometric center of the sphere, therefore leading to the expected result of the ball continuing in a straight line.
You are exactly right. As I mentioned in my response to @jadegecko the center of mass of the slinky/ball system will move in a straight line once released, but it won't be tangent to the outer circle, it will be tangent to the circle of the trajectory of the center of mass. So the straight line trajectory of the ball tangent to the outer circle IS an approximation to what actually happens.
This is like saying Galileo’s x ~ t^2 finding was actually wrong because of air friction… Bottom line: Answer b was and is the correct one.
I agree. Good thing that David didn't buy into A) or C)
@@oo88oo Exactly. This isn't a "surprising result", it's a pedantic trick question. If you're trying to demonstrate physical laws about circular motion of "a body", you use a string because it's necessary to apply a force to "the body" for the demonstration. He uses that necessity to smuggle in a concealed fact about the "body" that we assume we're supposed to be considering. He made the "force" part of the "mass of the system". Congratulations. Newton's Laws of Motion haven't been broken and B is still correct.
My thoughts exactly - this was a "gotcha" video that leaves me disinclined to watch others from this channel.
Since we're not ignoring small details, the ball also has to rotate. We can explain it either as to conserve angular momentum since it will no longer be rotating, or because points in the ball have different speeds since they are at different distances from the center of the rotation.
Amply demonstrated by the puck on the rotating table.
The ball might also be subject to libration (pendulum action) in one plane or another. Libration would cease but the ball's rotation would be slightly different depending on just when the "pull" stopped. I think.
they would need to try it with magnet "gravity"
but they would probably cheat different way then as well
@vibratingstring
Please note that I'm not out to start an argument. But I think that part of the point of this video is that "in the real world" every "system" has this "kind of delay". And that the length of the delay only differs in magnitude but never "goes away" completely.
And the "shortest" delay You can get can be calculated by measuring the distance from the "origin of the holding force" to the "released(point)object", and dividing that distance by the speed of light.
And I think this can(should) be said to apply even to "ideal systems", it's just that in most cases it's usually explicitly stated (in one way or another) that the "delay" and other real world factors that have "minor effects should be neglected" (e.g. air resistance, loads not being "points", etc.).
But that doesn't mean that these "minor forces" don't exist in the real world, rather just that they can be ignored in strictly hypothetical cases.
Best regards. t
@vibratingstring did you watch the very end of the video?
Interesting perspective - I once did an experiment similar to this but I used a 9" nail with feathers on it like an arrow and spun it at high speed by hand at about ten feet of line. I had set up a knife so that when I wanted to "Release" the nail, I would drop down a little at the knees and let the string be cut by the knife near the nail - worked great and it really was traveling at high velocity and I believe that the tiny speck of fishing line left beyond the knife was so minuscule that it had very nearly ZERO effect on the "STRAIGHT" trajectory of the nail - Your ball that continues on the circular path is interesting looking but, in effect, is simply NOT yet truly released from the force holding in in the circular pattern. The sliding puck was a similar case because - although the puck lost enough friction to slide; it was still, partially, being restricted by friction.
correct, he frames the question disingenuously .. aka, clickbait
That's very good clickbait.
He claims that the ball is, shortly, continuing the circular pat. And yes, when the total system of the ball PLUS it's fixture are released AT THE CENTER POINT of the circular movement, the ball itself won't immediately move in a straight line.
Fysics tells us, correctly, and repeatedly measured, that, when the ball, moving in circular motion (mark that the circular motion is measured in a system that is NOT rotating with the ball)
So a ball moving on a circle, held on the circle by anything, and IS RELEASED FROM THE ANYTHING, the ball IMMEDIATELY stops changing its direction. That is: from the very picosecond when the ball is freed from its centripetal force, it continues moving but in a straight line, tangent to the circle in the point where the ball was released.
In this vidéo, the ball stays linked to its SLINKY. It is not released from the 'slinky'. So that system leaving the circular motion is ball + fixture.
So
Release the slinky
Slinky+ball have a centre of gravity, that before the release was moving on a circle, and suddenly, immediately, without any delay, starts moving in a straight line .
I don't know WHO did that type of experiments. I only cannot imagine that some physicist and/or engineer didn't measure this.
If David had let go of his sling, he would have hit Goliath with the leather line instead of the stone.
But he released the stone. Somehow detaching the stone from the sling. At the right moment when the tangent was pointing to Goliath's head.
Bingo.
The stone, in a straight line, hit the giant and we read the happy ending of the story.
If David had released the sling, not the stone, that (loaded) sling would have flown away. It's centre of mass going in the straight line. Leaving its circle (smaller than the circle where the ball was circling) on the known tangent.
Why does he pretend that would be different for his contaption? Where in this video does he measure the path of the centre of mass of what is released?
Maybe
The standard problem is solved, so it isn't a problem any more.
If the ball
Agreed that the issue is the 'friction' between the puck and the table. It allows an unbalanced Centrifugal / Centripetal forces because of the friction! The assumption that the 'friction' is equivalent to instantly releasing the puck from the tether to the center is completely false.
Imagine this test done on an 'air table' with the puck circling a center point on the table with a rigid arm. Also, there is mechanism to release the puck instantly from the bar at any moment.
Now, the puck is forced to move in a circle because of the bar. i.e. their are no unbalanced centrifugal / centripetal forces.
What will happen when the puck is released?
The centrifugal force ends instantly as does the centripetal force. What is left is the horizontal motion. It really will be straight line motion at a tangent to the circle.
And by removing the Sun he says earth 🌎 doesn't fly away for 8 minutes. Can information travel faster than the speed of light. I no then it's simply not true
@@johntivatyi1569No, that's true.
Excellent video. It boils down to definition. When released from the centre, then you're no longer talking about a ball because the object is a [ball + string]. You'd have to release the ball at the radial end of the string to remove the string from the object, and then you will get answer B. It would have been good if you replaced the string with a metal rod with a release mechanism at the end to show the trajectory of the ball on its own.
Agreed, I think this would be more interesting. Presumably the direction of the ball would be tangential at the point of release, but would the ball have some spin and would the mechanism of release cause problems?
I suggested a hollow ridged piece of wood or plastic pipe with the wire passing down the middle to a releasing device inside the ball.
Or how about using a thread (or fishing line) and then take a very very sharp knife and insert from above into the plane of rotation near the ball. As you say with the slinky or the rubber tube the radial force on the ball is delayed by the tension propagation time (just like change in the gravity field on a planet if the sun vanished). The visual effect is interesting though. To say that people get the answer wrong is a bit disingenuous because the physics question is typically for an idealized world where the tension prop time is infinite. But I suppose the goal is to create a bit of drama.
The circular motion after release is because there is still tension for a small time even after release, use the release at circle, not at centre then result would be different
The question is more correctly posed as a linguistic one, rather than a physical sciences one. It boils down to how you define "release", because technically speaking the ball doesn't leave the [ball + string] system until the tension wave frees the ball - at which point it does precisely move tangentially to the point of "release".
This is not a circular motion effect. It's a [ball + tensioned string]-system effect. Dropping a ball or spinning a ball are simply different ways of arranging the [ball+string] system.
I always cut my "massless" string at the ball. While I think your presentation of the question is disingenuous, I do love the video and how it sheds "light" on the finer details. Don't forget the mass (or more appropriately, the moment of inertia) of the string, this will have a similar effect without retardation (change your slinky to a solid rod). I stand by the answer "b", but admittedly to the question of motion after breaking/cutting the string at the ball. What I really like about this video is that it make one think about the real life impacts of the fictitious assumptions/simplifications we employ and quickly forget about.
Well, to me the term disingenuous has a rather negative connotation, as if I was trying to pull a fast one. And I wasn’t, really. Yes, I knew people would misinterpret the question, but the point is not whether (a), (b), or (c) is the right answer, the point is to watch the ball continue in circular motion after the string is released! Anyway, I’m happy to hear that you still appreciated the video despite my choice to let people misinterpret the question.
Thank you. I also commented as he confuses everyone from the start and doesn't define things. But maybe that was just me vs 99%
@@AllThingsPhysicsRUclips Hmm. I'm thinking the negative connotation is not entirely unwarranted.
@@AllThingsPhysicsRUclips What if the "string" were completely inelastic, or rigid? Would the propagation be essentially instantaneous? I suppose the "string", being incompressible, would exert some additional effect, and perhaps how the result would play out would be influenced by the details of the release mechanism and the method of attachment of the "string" to the ball.
It was taught that way to me too, because it was meant to show what happens when the acceleration toward the center stops. If you leave the string, or slinky, attached it's a more complicated system. I don't think he was being disingenuous, just pointing out a problem with how the problem is presented by a lot of people. I've heard and read it presented both ways, my old physics teacher from high school presented it the correct way: what happens when the ball is disconnected, not the ball and string.
In a slingshot the ball is released from the ‘string’, so the expected path, a tangent to the circle is correct.
I'd just like say that the production on this video was outstanding. The clear writing, your eloquent narration, the beautiful graphics and slow motion edits to demonstrate your points. I think if you can pick the right topics you will have a million subs before too long.
Wow, thank you! I hope you’re right! And I’ve got a fantastic list of topics just waiting to be made into videos! Stay tuned!
@@AllThingsPhysicsRUclips Isn't the net force on the ball zero in both cases? Both when it is hanging vertically and when it is being rotated on the turntable the speed isn't changing. It's a constant radial distance from the center when in motion around the circle, and that doesn't change until the inner end of the spring reaches that radius and goes beyond it.
@@AllThingsPhysicsRUclips My first impression was that it would be the usual sum of orthogonal vectors ie of the tangential and radial vectors.
@@AllThingsPhysicsRUclips More slinky material! I came home with a box of them my junior year at Drexel. But we didn't have cameras in 1978. It must be so great in school today.
This was great, thanks.
@@joelwexler You don't need to be in school...you can continue to learn/figure things out for yourself right now! Never stop learning!
Thank you. I'm a retiree with a PhD in physics. You gave me something new to think about. I had a teacher when I was a student, Prof Brian Pippard (en.wikipedia.org/wiki/Brian_Pippard), who loved to demonstrate simple physical systems that gave unexpected results, such as spinning potatoes. Or how to use a glass of milk, a laser and a pencil to measure the astigmatism in your eye. I class this video as being in that mold.
Glad you enjoyed it! I hope you'll consider subscribing and sharing the video with others!
As a young boy, I made and practiced throwing the bolas. I'm wondering if some of these same properties affected my throws? Had a lot of fun all the same.
@adrianstephens56 I don't see the misleading point about "circular motion" being in the same class as your experiments, since the latter demonstrate genuine and interesting effects, whereas the idea of this video is to mislead people with an abstract diagram -- signalling the usual abstracting away from nasty real-world effects, such as air-resistance, non-instant releases, wobbly centrers of rotations, etc. -- to essentially claim that the correct intuition most people have about circular motion is wrong. The intuition is not wrong, all this video does is to point out that the idealisation of an instant release propagation does not exist in the real world. That's a remark on the property of materials and (a point not made in this video) the limited communication speed between cause and effect, it is not a remark on "circular motion". I like your experiments.
@@coolcat23 Yes, but this is a physics video not a mathematics video so it's not so bad that it does that. I agree though, it doesn't sound as interesting as what the original poster mentioned imho.
@@coolcat23 I agree mostly but the propagation is very easy to overlook/miss. And I'd say in the real world, lacking this intuition could be a problem, as explained at the end of the video.
Its always nice to know an event as catastrophic as the sun disappearing could take place and I would remain blissfully unaware of it.
But only if 8 minutes is longer than the rest of your life, so there's that.
But I was rather shocked when I learned that all forces, including gravity, only propagate at the speed of light. We didn't discuss that in a year of introductory physics in college (which is all I had).
So while gravity is nonlocal, it isn't simultaneously nonlocal (I ran into this as an adult (layman) casually trying to understand how gravity could be nonlocal AND have particles as the mechanism to control the force.
More than that, I was thinking, what if the sun disappears, as explained for 8 mins, I will be oblivious as the earth keeps on rotating... but say, if the sun re-appears after 5 mins, what will happen? Will earth continue to rotate? There won't be any issues with earth's motion because of sun's disappearance for full 5mins?
@@varunkashyapv8383 Completely theoretical but probably: Earth would continue its orbit until it was no longer affected by the Suns gravity when it will move tangentially to its orbit in a straight line. By the time that the sun reappears, the Earth would have moved about 9000km which is not much compared to the fact the distance to the Sun varies between 147,100,000 and 152,100,000 kilometres however depending on where in its orbit the Earth was, it may just result in a very mild change to our orbit without noticeable effects to a substantial change to the min and max of the elliptical (either more elliptical or less)
As others have said. These kinds of questions assume idealised scenarios. Almost like the string vanishes from existence (like the sun in the final example). So B is the 'correct' answer. Great expansion of the concept. The falling slinky alone is a great challenge to the assumptions! Love this video
Ah, very thoughtful response.
One principle of classical physics calculations is that one can use the coordinate system that gives the ease of calculation that is desired, and the result will be the same from other "correct" analyses in different frames when translated back to that frame. In this case, we also need to consider the assumptions of the problem as Darren notes. In physics class we would usually suggest the string was infinitesimal or insignificant mass. If not, then that needed to to be specified as part of the problem description.
If assuming infinitesimal mass frictionless string, then the Cartesian frame is the easiest frame to use. Then what is meant by "release"? If instantaneously discontinuing the force or tension on the string, rather than letting the string slip slowly out of one's fingers, then very simple indeed and "b)" is clearly the answer because ball's motion is -Y direction and no forces to modify that.
The drawings idealize over the actual motion of a person swinging the ball, because that itself would most likely be a non-circular motion because the person is not moving in a circular hand motion and all the interactions involved having stabilized. So the drawing belies the actual video example. If we are following the drawing, are we not using simplified assumptions like circular motion, so why not also infinitesimal string mass and instantaneous release?
The video segment with the puck on a rotating disk is sort of a non-sequitur (though interesting). The point there is that after beginning to slip -- a matter of non-linear frictional forces that are reduced when the tendency to "stick" is overcome, are nevertheless forces imparted by the rotating disk upon which the puck is moving, which explain the partial spiral motion.
Now a similar case might be rigid rod with significant mass replacing the string, but circular motion and instantaneous release. In this case the center of mass of the rod/ball system is useful, and also probably Cartesian frame. Then the rate of rotation will be constant at the moment of release, and the center of mass should follow path "b)" but of course not the ball but the CM. So a translating frame following the CM is useful, wherein the motion will be strictly circular, but then translate to the still frame to see the compound motions.
Then extending what I said, the Slinky (tm) tends to demonstrate a transmission line effect (traveling wave). So at point of release, the change in forces are only apparent at the center part of the slinky system. In that manner the tension forces on more outer portions remain in effect until the traveling wave reaches that portion. Here a CM frame would again be useful, and would indeed the CM should follow the -Y path (as 'b') but the object is not rigid so the motions within that frame will include the traveling wave effect and be quite complex.
Then in the end of course any real string is not only not massless, nor frictionless in air, but also has a modulus of elasticity so that it actually can be modeled along with its mass density as the same as the Slinky, but the wave travels must faster. Thus in the end, "a" is the answer for any physical string -- but the time frame before looking more like CM frame above would be very very short.
AND of course -- the CM frame is not completely accurate because the air, presumably stationary to the reference frame, puts force on the ensemble and slows the overall frame and effects the parts.
It just has to release, like that spinlaunch system.
@@gordonelliott7870I think repeating the puck-on-a-surface situation, but immediately removing the centripetal force by halting the turntable with a hard stop would come close to simulating the instantaneous release of a string. The puck will, for a short time, continue to move in the same direction it was going when the table was braked down to 0 very very quickly.
Good presentation and the slow motion video with the slinky spinning the ball to illustrate the effect of finite time required to propagate the information was very impressive. I was thinking of the "what if the sun disappears" case as an example but you mentioned it at the end. That being said, the question in the beginning was tricky in the sense that most people would assume you are using "fully rigid" spring. Could have started off by clearly stating that the string is elastic, or by not even posing this question but just saying that this video demonstrates the effect of elasticity or finite speed of propagation of information on circular motion. That alone in itself is incredible in itself, as you have demonstrated in the rest of the video.
This tricky question at the beginning made it hard to take you seriously in the beginning, especially when it was immediately followed by the example of an object on a rotating turntable wherein the cause of the centripetal force is frictional force and the behavior you showed (it slipping and it following a curved path) was for a phenomenon not directly related to this topic, which you didn't even get into in the video! Could have avoided the sensationalism, just my opinion.
There are so many amazing things we find when we look at how physics really plays out over what we think.
Very well demonstrated. I really appreciated the fact that you reiterated the importance of this by giving examples of how it could actually effect the outcome of systems.
The makers of "Spinlaunch" should watch this.
@@Lt.-Dans-Legs No one can know everything. Even when their life depends on it. Your ridicule is unwarranted.
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. The anti gravity is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
You have an incredible talent for making physics videos that are engaging for physics novices and experts! I can't wait to see what else you do with this channel. 😊
A note for physics teachers: standardized tests (AP Physics, MCATS, etc) will expect your students to give the straight line answer (B). This is typically because instead of asking about the string being released, they ask about the string being CUT, and the assumption is that it is cut at the location of the ball, so there is no delay while a wave propagates. If your students are comfortable with the material, be sure to point out this important difference.
Thank you for the kind words, and for the important disclaimer regarding tests. I think it's fair to say that ALL test questions on this topic assume that the force goes to zero instantaneously, for example, but cutting the string right next to the mass. It is definitely important that students be aware of the difference!
I think it is more subtle than that; really, in introductory physics we basically just assume that changes propagate instantaneously, rather than doing the messy business of taking propagation time into account. I think it is probably a good idea to point out that we are making this assumption, and how it simplifies what we are doing without substantially changing the physics involved.
Ultimately, the question is trying to ask what happens to something in circular motion when it no longer feels a centripetal force. For the example of a ball on a string, the ball no longer feels the centripetal force only after the information has had time to propagate along the string, but that is close enough to being the same moment the string is released as to not matter in most instances.
@@joeo3377 kind of low how in real life everything has inductance and capacitance
@@AllThingsPhysicsRUclips ... The wave propagation speed in ideal strings is infinite because a) they are infinitely longitudinally rigid (Hook spring modulus k is infinite) and they have zero mass so the acceleration for any force applied is infinite which in turn leads to infinite speed.
@@csours Indeed, and that is messy business!
There are a lot of comments complaining about the question being misleading, but I feel like those people are way too concerned about being right and how that affects their own egos.
In reality, the video isn't about if you already know the answer, it's about learning something new, or thinking about things in a new way, and it does a fantastic job of it. It's very well communicated, a very solid length for this topic, and has a very good mix of theoretical and experimental sections. Great job and I'm looking forward to more videos!
Thank you so much for this comment. It mirrors my own thoughts quite well!
It is truly interesting, that is for sure. I assumed 'C' wrongly, and now I'm wondering exactly where in their rotation a hammer thrower (sport) releases the hammer.
Also how a stretched rubber sheet can be used to represent space-time; not with objects stretching it downward, but by objects stretching it inwards, towards themselves.
Wouldn't your comment be about your ego?
If the response to this video has been negative despite it being informative, then it is only because how the video phrased its central question. Essentially they are making it a trick question, to create a "gotcha!" moment. This is cheap and off-putting.
"[I]t's about learning something new..." I think the same thing when reading comments on Einstein videos.
As an Electrical engineer, I only did physics in my first year of University (freshman year, but I don't live in the USA). Our physics, statics and dynamics problems always had disclaimers like a "light rope," "ignoring air resistance," "rigid bodies," etc. Therefore my answer was also B.
I think it's B too.
Where he gets you is his question , "what happen to the ball when I release "the string." and on this he's right, but as in his example of the 12.6 cm deviation that only equates to 1.26%, almost hardly worth mentioning unless you're slinging rockets to space via a rope or calculating the theoretical release point when David binged Goliath in the head and won the day.....lol. Mostly your right "b" is the right answer.
Your answer was correct. This is about semantics games, and the circular motion is about the SYSTEM, not the ball.
You really can't DO elementary physics problems correctly without such disclaimers.
I remember being VERY PISSED OFF during my first physics test, where my (excellent) physics teacher gave a problem that was very hard to think about given the reality of friction, in the context of a simple Sum of forces in two dimensions problem.
After literally 40 hours over the weekend of working problems to be prepared, I almost confused myself and messed up the problem (he always gave unique problems unlike those in the book / lectures to ensure people knew how to THINK vs. memorize).
I did calm down, and tell myself to trust the principles and techniques I had learned well and studied, and worked the problem correctly. Then, later in his office, we had a chat about clearly stating such assumptions, which he agreed with.
Another way to answer the question at the outset: At the moment the slinky is released, the system comprised of the ball and slinky COMBINED will have a center-of-mass trajectory that is straight. The motion of the ball plus slinky is not shown in the video, but it will be a tumbling motion around the uniformly moving center of mass. The system (ball plus slinky) is free of external forces (ignoring gravity) from the instant it's released. On the other hand, the ball feels a force from the slinky. The mass of the slinky is essential because it provides the inertia that allows the end of the slinky connected to the ball to maintain the tension that it had before it was released. That tension is of course the centripetal force that kept the ball on a circle before the release, and it indeed continues to force the ball onto a circle for some time. The tension at the outer end can change only if the deformation of the slinky slightly further inward changes, and this can in turn only happen if the tension further inward from that portion of the slinky changes, etc. The change in deformation at every location is an acceleration that happens with a delay dictated by the inertia of that portion of the slinky (Newton's Second Law). As always when inertia and tension of a medium compete, the outcome is a wave.
I agree. You can really see this if you track the approximate COM from 12:52 in the video.
This was just a grifter making click bait. The slinky is NOT analogous to a string at all. This should be obvious.
@@anyfriendofkevinbaconisafr177you are 100% wrong.
Of course the slinky is analogous to the string, and the fact that you don't understand that, makes it obvious that you don't know all that much..
"The motion of the ball plus slinky is not shown in the video, but it will be a tumbling motion around the uniformly moving center of mass."
You can see it very briefly at 7:52.
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. The anti gravity is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
Very nice!. As others have said it shows the importance of those words and phrases used to make elementary problems tractable: '...light, inelastic string...', '...a point mass...`, '...a rigid rod...`, `...rolls without slipping...` or whatever. Good to see videos like this that show how removing these kinds of assumptions has measurable and often surprising effects.
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. The anti gravity is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
@@vincecox8376Please either see a psychiatrist or a physicist - what a load of nonsensical bullshit - what exactly is special about center of the magnetic field, how exactly does magnetic field and physical vibration are supposed to interact in such a way to break laws of quantum mechanics that govern physical material properties of an object and why exactly granite, that which is an amalgam rock..?😂
@@MargoTheNerd Sorry your the one that needs help. I've been in the field of physics for over 50 years, I have many credentials , Please get yourself some mental health. I can put money where my moth is,,, I know exactly what I. talking about, BET YOU $10,000, on any of my points....
A cosmological analogy to this is that if the Sun was to suddenly poof out of existence, then the Earth would in-fact continue orbiting around the sun for about another 8 minutes. Essentially, in any physical system, there is a speed of causality of some sort - The information about "the ball was released" *always* takes some time to move from point A (the place of release) to point B (the ball). For the Earth rotating around the Sun, that speed is the speed of light - the ultimate speed of causality. This video is a cautionary tale to always make sure that the mathematics makes physical sense if you apply mathematics to physics.
Heh. I guess you didn't watch the video all the way through when you made this comment. I talk about this at the end of the video. Please watch again! 😊
@@AllThingsPhysicsRUclips Haha, I clicked off *literally* the second before that clip.
@@BirdbrainEngineer Then you probably missed the giant slinky as well! 😂
@@AllThingsPhysicsRUclips I did see it when I was checking the end again :P
But again the sun rotates around the earth. The earth is stationary. NASA even accepts this but lies to the public.
Wave propagation is a fascinating thing, and is involved in surprising processes. It appears that it is involved in the swinging of a ball on a string (or slinky) in a circle...at least upon release. If you looked carefully, you could actually see a small reflected wave traveling back up the string when the tension wave reached the ball. The impedance of the ball is quite high compared to the string, resulting in the reflected wave. Same thing happens with sound waves, radio waves, and even water waves. Fascinating stuff.
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. This is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
drive.google.com/file/d/15qCtZTSe-GuGbEbuc7NmGh5Pc9T9rhN1/view?usp=drivesdk
@@vincecox8376You may want to correct your English to promote a fringe hypotheses.
"Non" not none. "A lot" not allot. "Spread" not spred. "Huge" not hugh.
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. The anti gravity is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
Agreed, really fascinating. So you have the force propagated out as a slow moving signal, which doesn't update and tell the ball what to do in accordance with physics until it arrives, that's one picture. But then there is this second picture which is more local, about the center of mass and equilibrium and it just works out to match the first picture. Really mind blowing video to me.
Engineering with drag, elasticity, and mass, vs. Mathematical Physics, where to demonstrate gross effects of one activity most clearly, you can discount all other activities. So, definitely a trick question, when you pose it in simplified terms, then measure in the real world. Because if you had included, "on a string with mass and elasticity, through a medium with drag," only a few of us would even hazard a guess. So apart from the intellectual dishonesty, a great experiment to show the quite interesting real effect.
I watch a lot of science channels and this was such a unique video. I havent really watched anything on classical physics and forgot how interesting it can be.
That is mind blowing, you have to wait for the tension wave to reach the ball, because until then the centripetal force is still acting upon it... and obviously the less time it takes for the wave, the more answer b becomes a closer approximation of what happens. Great video!
You got it!
I think it's a cool concept just a bit misleading with the initial multiple choice. The assumptions in the problem setup matter too much.
B is only wrong if you assume the string isn't ideal, which I thought was implied. I took the question to be, "What happens to the ball once the effect of the string is no longer acting on it." Which would be B. But he really wanted to nitpick about the instant between the string being cut and the effect reaching the ball.
On a practical level, the speed of sound is very fast in a string, so B is the best approximation of the path when not using a slinky string model. The slinky deliberately exaggerates the effect beyond what a typical string would exhibit.
Assuming the string is ideal, and the equation for the speed of sound in a string being, sqrt(Tension/linear density), the speed approaches infinity as linear density approaches zero.
Like he said, it's subtle, but it's important to be aware of the assumptions that are made in solving a problem. And when asking a question to leave as little to interpretation as possible.
@@RubixB0y yeah totally agree
@@RubixB0y and @allthingsphysics "I think it's a cool concept just a bit misleading"
This Video does not show, that answer B is wrong.
It shows (in an very impressive way!), that the speed of propagation of information (and thus causality) makes an enormous effect.
In the moment, the information reaches the ball, it will move on the tangent.
So answer B is correct.
But when causality is transferred via the slinky, causality propagates very slow.
As long, as the ball "does not know" that the other end of the spring was released, it will not change the trajectory.
@@amuller3101 I don't think it's misleading because I answer the exact question that's originally posed. Yes, I knew that many people would misinterpret the question, and I relied on that to make things interesting. But that's not misleading.
I'd have put he house on B!! It was fascinating to see this - especially how the slinky made it easier to see
definitely a bit of a trick question. shows the difference between "releasing the string" versus "detaching the ball from the string". Under most circumstances the mass and momentum of the string would be considered negligible, but if its not then these two scenarios are completely different for the unintuitive reasons demonstrated in this video.
Hmmm...not sure I'd call it a "trick" question, given that I provided experimental evidence in three different systems, but no doubt I am focusing on a bit of a technicality. And technically, this doesn't really have anything to do with the mass/momentum of the string because the same thing would happen if there was a ball connected to the other end of the string. It is simply due to the finite travel time of the wave speed (which, admittedly, probably depends on the mass density of the string).
@@AllThingsPhysicsRUclips Sorry, I wasn't being precise in my language. In most simplified physics problem it would be the tension wave of the string not being considered, not the mass and momentum. That was how I should have worded it. And my first point still stands. This phenomenon would not be present if the ball were to be detached from the string/slinky at their connection point, rather than the string/slinky being released from the center of the circle with the ball still attached.
@@ThenameisMarsh You're absolutely right that this phenomenon would not be present if the string was cut right where the ball is connected. And I've seen the question posed exactly that way. But that's not very realistic; it's certainly not the way most people would actually do the experiment.
It's not a trick question. It is a challenge question meant to illustrate systems are complex. I first encountered this in college: the Earth does not travel in an exact elliptical path around the sun. The moon has mass, it and the Earth wobble during orbit. The center of the system is what does not wobble. @@AllThingsPhysicsRUclips
I would have liked to see a ball releised on the end of a string after the demonstration with the elastic, I get that it isn't really nessisary for understanding but would have "closed the loop" if you will in terms of explaining the topic. Very cool video.
Yes, unfortunately it would not show anything. The wave propagation at (approximately) 2,000 m/s, coupled with the lower frame rates we had to use because the camera was so far away, just made it impossible. I thought including that footage was simply not worth it.
@@AllThingsPhysicsRUclips But if you release the ball at the end, rather than release the tether, why would it continue on a curve path? Isn’t the curved path resulting from the tension equilibrium that then dissipates outwardly whereas by releasing from the ball end, the dissipation would occur centripetally and then you would see the ball take a tangential path?
@@AllThingsPhysicsRUclips move the camera closer and time the release. You get a short section but you might still be able to trace a curve (depending ofc)
The reason the ball seems to curve when it's attached by a Slinky after the Slinky is released is that the ball is in a neutraly balanced state and position. This is because it is actually being held in place by the resistance of the Slinky coils. Which is countering the centrifugal force equally, thus equilibrium.
Therefore, C was the correct answer.
Got me with that one. When the connection between the ball and the string is released, the correct answer is (b)... But the experiments were great anyway!
The words "immediately on release of the string" do not mean the same as "when the ball is let go". The ball is still under the force of the tension of the string all the while until the propagated wave reaches the ball. It is at this point that the ball is "let go" and then it turns out that B was indeed the correct answer, at this point in time.
Interesting! I may have changed some things a little differently in your ball string experiment, but only bc you shared the set up and results of your tests!
I appreciate all the thought, effort, and execution of the experiments!
Well done!
Thanks so much. I hope you will consider subscribing snd sharing the video with others!
The puck on the rotating table is governed by one other force, not mentioned in the early part of the demonstration. It is held down onto the table by friction. This changes the whole game because it can't possibly behave like a weight on a string. 5:16
In high school physics, we saw the same experiment performed where the rotating weight is an air puck on a steel table. Essentially frictionless. So when testing was quickly severed by a flame, the weight indeed left orbit in a tangential direction.
I wasn't crazy about the puck description, either. It seems to ignore the friction that still exists once the puck is moving (kinetic friction), and if you account for that the puck isn't a good analog for the ball on the string. I think the video would be better without it.
@@michaelporter1 And swinging a ball around his head produced and elliptical path, _not_ a circular one. False premise leads to false conclusion.
Ah well; so much for post-truth physics, huh?
This is wonderful. I like that the "string" continues rotating about its center. And the connection to the gravity conundrum about the disappearance of the sun. Thank you
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. This is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
@@vincecox8376 Err ......
Who here is talking about antigravity, or ANYTHING you mention in this comment. Try to make comments about the material in the vid. No one is impressed
12:20 that blew my freaking mind 🤯
Awesome! I like to hear that!
We were presented with this question by our university physics professor back in the early 90s. But in our scenario, the ball was released from the end of the string. So we determined that the ball would continue in an arced path between B and C, due to the centrifugal force and angular momentum.
And this is the truth of it. The premise of this demonstration was disguised in the introduction to the problem to mislead. It was obvious to me that your solution was correct, but it was not offered as a "solution". This allowed them to introduce the elasticity and reframe it as "briefly" and "subtle" during the discussion. Not impressed.
Angular momentum is conserved by linear motion, and the centrifugal force is zero after the ball is released.
I don't think you can talk about centrifugal force in an inertial reference frame which I assume is the one we take when we try to explain this. Also I think angular momentum is perfectly conserved on a straight path as well so I don't understand your reasoning. If your answer were correct, what would provide a non-tangential force to change the direction of the movement of the center of mass? The only way I could see rotation playing any role is if there were some aerodynamic effects of lower pressure on one side ,thanks to the initial rotation, and airdrag
Really interesting video and presentation,
Stil, I"m a bit puzzled by some things into the video.
1°) I have noticed that the tiny upper disc when you increase the speed of the downside sustaining rotating disc; not only at a certain speed start to glide radialy; but also spins on itself (seen on the larger disc but also after passing over the edge).
2°) When you think about the problem, you may see it:
-As letting the rope and the weight go at a given time of the rotation; as such the tension into the string is proportional to the weight and to the number of rotations per second (rpm); then you may consider the string to be a spring elastically proportional to the tension; when releasing the rope, the weight continues its circular motion until the contraction wave (tension) from the rope hits it; then the weight and rope follow the radial trajectory. But this reaction is so fast that it must not be visibleon footage.
--As a stone from a spinning sling, and this usually go straight radialy to its target when set free from the sling.
Regards,
PHZ
(PHILOU Zrealone from the Science Madness forum)
This is well postulated though true in only a limited survey, the most directive force is the awareness, of the observer observing.❤❤❤
Excellent video. I did not think of the string tension to have a material impact. Impact of this issue was really striking with the slinky. Thanks for making this video - learned something!
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. This is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
@@vincecox8376 What the heck has this to do with the material in this video? Really, you are all over the place with this same cut/paste comment
Yadda yadda
Kind of obsessive, methinx
Awesome! This video very much feels like the an early Veritasium or Steve Mould... perhaps Smarter Every Day-esc kind of content. In the future, you might want to put a small watermark in the corner of the video where the experiment is taking place (the automatic RUclips watermark is a bit small and does not contain the channel name) - that way most people that share the most interesting part (the actual experiment demonstration) will be helping to signal boost the channel :)
Interesting idea about the watermark. I suppose I should be doing that, but I'm still pretty new to all this and I've still got a lot to learn.
@BridbrainEngineer -- it pairs really well with the one Steve and Destin did together called "Which Way Will the Water Go?", about a spinning sprinkler, since it deals with the (non)intuitive nature of switching between rotating and non-rotating reference frames.
thank you @pataplan! All the experiments you've performed DO NOT duplicate the situation framed in the question, which is assumed, as noted by @pataplan, to be a detaching at the ball end, and so there is no elasticity effect, which could be though of as a centrifugal force holding the ball in a circular path.
but interesting to ponder!
We’ll, in my defense, I did ask quite explicitly what happens to the object immediately after releasing the string. So the situation framed in the question is precisely what was investigated. 🙂
@@AllThingsPhysicsRUclips Although the wording was imprecise/ambiguous.
Excellent.I can see its important now to distinguish where the release happens- at your hand or if the string breaks at the point where the ball is connected.
This is why I consider the presentation to be more of a trick question than a "surprising result".
maybe not 'trick' more like limited or ill defined...@@TexGuvnah
Thank you so much for this video. I was pondering this exact question a few months ago and had also followed the logic of the slinky drop. My final question in fact was what would happen to earth’s trajectory if the sun was to suddenly cease to exist. Great demonstration of the physics of it all.
You were pondering this question? Alright then
"What path does the object take immediately after releasing the string" is still "b", because it's the object that releases the string, AKA, a slingshot. What you are actually talking about is "What path does the object take immediately after the string is released".
How does an object release a string? What path does the object take immediately after the string is released [by the person swinging the object around!]
@@AllThingsPhysicsRUclips "how does the object release the string" - the object tears off, or release mechanism attaching object to the string is triggered. Slingshot.
as usual, great vid! that was a really interesting concept and your presentation was both intuitive, and thorough :D so glad to see you back!
Thank you. It feels really good to be back, and hopefully I'm back for good now!
Man, just blown my mind. I didn't know that from my college. Also graphic quality as well as briefing is Extremely good. Keep up the good work
Thanks a lot! I hope you will consider subscribing and sharing the video with others!
These days, it is comforting to hear exact language. Thank you.
Weirdly, I think for many people who have some knowledge in physics, the earth-sun example is actually the most intuitive.
That was one of thE coolest demos I ever seen ..... Awesome Channel !
Glad you liked it. Please share with (many) others!
Very stimulating, thanks. That is why in many statements of this problem is said that the string "disappears" or "disconnects at the ball side" (a relatively simple mechanism to do) so students focus on the main question. Even with a rigid bar this would happen just the sound velocity in the bar would be so fast that we could not see it (Ok there are come complications like the mass and inertia moment of the set ball-bar though). That is why some say physics is the art of ignore all the complications and focus on the essential (or like a professor I had used to say anytime you see a potential well approximate it by a parabola because we only want to solve for simple harmonic oscillator, we add complications later aka here there is more light). Imagine if Galileo in his experiences did not extrapolate the friction out: he would still be stuck in Aristotelian Physics. That is why we only tell that to students after they are mature enough to factor all the complexities.
We'd like to see a release mechanism on the ball end of the string. That would better match our early expectation of where you were going to take things.
Or a stick.
i think it all drills down on how the effect travels throughout the system...
The wave propagating along the string transmit the information that the string has been detached. As long as the information has not reached the ball, the ball must continue on the exact same path as if the string was still attached at the center.
Congratulation on finding a medium with such a slow signal velocity.
At first I guessed B, then (as I expected) you said it was incorrect. After a little bit of thinking, I thought “oh is this something to do with the finite speed of the release of tension in the rope? Is it A? It’s A, isn’t it…” the issue is because of your lovely diagram I was imagining it in an ideal world where that lack of force transmitted instantly across the rope. But we don’t live in that world. Well played, you got me
Well, I hope you don't think I was trying to "get" you. On the other hand, few people would choose answer (a) and I think it's quite fascinating to see that answer (a) actually holds in a real-world setting!
The thing that leads people (even physicists) to go for answer B I guess is the unspoken assumption that "the string being released" means the centripetal force is immediately gone. We are so used to working with idealized models (infinitel stiff rods, massless strings) in which case that information is indeed instantaneously transmitted and the answer is in fact B.
Once you challenge that assumption it quickly becomes clear that the answer is A in a real world setting.
I fell into the trap of saying B myself, though I knew that was gonna be wrong, because why else would you make a video about it. Quickly realized where you were going with it when you started talking about the slinky.
Still though, you're right about how striking it is to actually see the effect! It looks so damn strange, even though you intellectually know that's how the motion must be.
Terrific video!
Yes, we (physicists) often simplify things to get across the basics, which is obviously the way you need to teach the subject, but once you've got the basics I find it fun to think a little more deeply about things we often take for granted.
@@AllThingsPhysicsRUclips Absolutely :)
In the question posed as a test, we were given imprecise information and told to use it as if it were perfect.
A ball swung on a string by a person is not going to describe a perfect circle, but we are expected to assume that it does act that way. Similarly, the wave traveling down the string is estimated to travel at the speed of a rifle bullet ---obviously far too fast for us to be able to measure and use in the problem.
So just as we are expected to assume that the ball is describing a perfect circle, it is only reasonable to expect that we will assume the instantaneous release of the ball.
So I suggest that for the purposes of this test, answer B is the correct answer.
It's really dishonest to add a lot of other conditions to such a test after the fact, in my view.
It seems like the mass travels tangentially to not the exact release point, but the tension release point, which makes complete sense.
Great video.
This is wonderful! I have taught physics for a long time, and this isa. very accessible extension of the core physics of circular motion. Thank you for presenting it so well.
So glad you liked it. Please consider sharing with others who might be interested!
I love your presentation style! Your excitement is so contagious, and the explanations are very easy to follow.
Thank you for the kind words!
Really awesome video, 12:20 I was struck with the realization of "phonon!" and wave propagations etc and it's really cool you could show that.
I'm a materials science undergrad (senior)
Glad you liked the video! I hope you'll consider subscribing and sharing the video with others. If you're a materials science major, I've got a couple of other videos coming up that I think you're going to love!
I was under the impression that you were referring to releasing the string at the ball end. In this experiment the rotating object (ball) isn't being released, it's a ball and attached line. In any of these examples, forces acting on the ball causing it to change its direction are present momentarily.
In the experiment where the puck is on the rotation table, the friction acting on the puck isn't immediately released either, as the puck is still in contact with the rotating table and only moves once it has overcome the static friction.
Can we see the experiment again with the ball releasing at the ball end?
Thank you David for this surprising result. The explanation became complex because of the slinky wave, the elasticity of the bungee, and the extensibility of the string. The experiment we performed for a science fair 60 odd years ago in Brighton, UK was much simpler: a lead fishing weight on some monofilament. The monofilament was designed to fail at the weight end as the rotational speed was increased. We only had cine film to record the event. Of course, we concluded that the path was indeed tangential because that is what theory told us would happen. Within experimental error, that is! Seriously, with a high speed camera and the fracturing monofilament, you would find the expected result BUT your video would have been a lot less interesting and thought provoking!
Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. This is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..
It is neat to see the ball remain in place for a moment as a stretched Slinky drops. It from the gravity force downward is balanced with the tension of the coils trying to collapse upward. So net zero movement. I wonder what the record is for making a ball remain motionless in the air? Cool demonstration.
As usual, impressive visuals and great explanations throughout. Keep it up!
Thanks, will do!
I think its improtant to clarify what "relaasing" the string means. If it points to the instant when the centripetal force disappears/weakens, then B would be correct, right?
In effect the question you posed is equivalent to "what would happen if the Sun disappeared". We would continue to orbit for another 8 minutes (im disregarding the difficulty to define an agreeable point in time for both Earth and Sun in which the Sun should disappear). The only difference is the wave is now gravitational and much faster.
the ball would always move in circular motion for a brief moment in time even theoretically as the tension wave even in a massless string can not travel faster than the speed of light
ergo the ball is very briefly in circular motion both in theory and practice
I love the combination of esoteric/flat-earth aesthetics and actual, real science being done. The contrast couldn't be larger. Really cool!
I'm guessing if the release happens on the ball-side of the string instead of the slinger-side of the string the straight tangential line would be accurate.
My gut feeling says if the ball was released with the slinky still attached, not only would it follow trajectory a as demonstrated in the video, but after the wave of the spring has released the tension that kept it on the circular trajectory, (again gut feeling) it should probably then actually start curving outward, basically again circular motion but away from the original circle. There is inertia in the string/slinky and that has to go somewhere 😅
17:49 Seeing your ending graphic with the *solar eclipse* just now is somewhat poignant and poetic, since I got to see totality of the _April 2024 solar eclipse_ from my back yard just a couple of weeks ago.
I saw the 2017 solar eclipse in totality in Salem Oregon and it was spectacular! That was the motivation behind my banner and logo. Glad you got to see the recent one. (I was in Killeen, TX this year and we lucked out with the clouds…not quite as spectacular as in 2017, but still quite awesome!)
Of course, if we want to be sufficiently pedantic, the ball can't swing in a perfect circle to begin with because of subtle distortions in spacetime. Physics is always a question of which effects are small enough to be ignored. Leaving an ever more accurate and pedantic series of "but actually's" as you take into account the heisingberg uncertainty principle on the balls position and the effect of the gravitational waves produced by the strings release on the ball.
You are absolutely right about that. But at least in this case I was able to provide visual confirmation of the point I am trying to make!
And yet, another yet. 🙂
Great video! Love your channel, keep up the good work, man!
Thanks!!
Thanks for this very interesting video. I passed my physics exams back at university, but this one I failed it signally
such a great video, the effort for all those amazing visuals and experiments is well worth it.
Thank you ❤
Certainly an interesting situation, yet I agree with @pataplan the proposition was a little misleading or ambiguous. I too assumed the release point was at the outer end of the string, in which case the answer would be "b". Your results were surprising, yet after thinking about it, it does make sense but only because of the elastic tension in the tether, as it continues to provide a force on the ball until it is completely dissipated. I would like to see what would happen if the tether was rigid, say a carbon fiber rod or something with almost no tensile elasticity, release it from the hub as here, and then see what happens. I believe the ball then would continue in a still curved path, but no longer matching the radius of its former circle. In this case I imagine the center of mass of the ball/carbon fiber tether combo would instantly begin moving in a tangential trajectory, however the ball and tether would now begin rotating along this straight tangential trajectory, around the center of mass of the combined unit, and thus the ball itself would oscillate in its path around this center of mass.
First video of yours I've seen, instantly subscribed and shall have to see what else you explore. This is the sort of stuff I'd stay in the classroom in high school during lunch time to argue/discuss with the other nerds! Love it!
Really learned something new. I almost freaked out when you striked out option b. But you convinced me.
Great video -- you learn something new every day!
I'm assuming that if the release point was located where the ball is attached to the string (instead of near the centre of the circular path), then we'd see the ball immediately follow the tangential direction. However, I'm curious about whether the ball would experience the Magnus effect at all soon after, which could curve its trajectory sideways. Since the ball is already experiencing some rotation while its still tethered (1 full rotation per revolution, similar to the coin rotation paradox), I guess I'm wondering if that rotation is preserved after the ball is released? Would it follow the same principle that you demonstrated in this video? Thanks!
Well, I'll be honest, I hadn't thought about that. But you are correct that the ball is rotating and I believe this rotation would continue. Thus, if we don't neglect air resistance then the Magnus force would certainly be acting, though the rotation rate is pretty small so this force would likewise be quite small.
Yes a release at the ball would more closely approximate the tangent as not only would it more closely approximate the ridged body assumptions of these algorithms, it wouldn't have the string in the ensemble, which needs to be considered.
While the Slinky's bottom doesn't move, the total mass of the spring is being accelerated as one would expect under gravity.
IMHO some of this is a product of the limitations of the Newtonian approximations, Euler angles, and the cross product.
In the GR case of the sun disappearing, the Earth is mostly following a geodesic, or the straight line on a curved surface. There would not be any perceived changes in angular momentum at all from the earth's reference frame. Just what is a straight line would change (ignoring tidal and other smaller effects)
It is useful to replace fictitious force with apparent force when talking about effects like centrifugal force in Newtonian mechanics and for the apparent force of gravity under GR.
These apparent forces are an artifact of the chosen frame of reference.
Choosing the ball's perspective when attached to a string, and not the string ball assembly as a whole is the cause of the apparent paradox here.
Spherical cows (oversimplified models) allow for practical models with simpler math.
Euler angles, the cross product, and the fact that SO(3) is topologically a 3-torus really move your above question into the Grassman algebra realm for intuition IMHO
" . . .you learn something new every day!"
Sadly, many Do NOT !
☆
Really loved the way you built up from slinky to string! Would be interesting to see the path traced by the COG of the entire released system, which may actually continue on a tangent from it's position at the time of release.. Thanks!
Yes, that is exactly what the COM will do!
That's not correct, again. 🙄 the COM travels rapidly up the string towards the ball after release. But since the string is still attached to the ball, we already know what happens to the ball+string system, as it's what this video is about.
But if you are to now claim that the COM immediately moves tangentially from the circle at the moment of release, then your whole premise of this video is incorrect!
If a much-more-massive string is used, the COG never reaches the ball, but stays along the string somewhere. Therefore if the ball+string COG system as a whole moves tangentially immediately, the net effect on the ball is **also** to move away from the circle. This would be very clear with a massive string. With a low-mass string it just **appears** that the ball doesn't stray from its circular path until the tension wave releases it from its tether; but actually it's merely that the COG change relative to the tension wave is not humanly perceptible. It has after already moved off the circumference. Using a heavier string would demonstrate this clearly.
@@nowandrew4442 I think you are mistaken. Consider the ball and string as the system. As soon as the string is released, then there are no forces acting on the ball/string system. Hence, the CM of the ball string system will move in a straight line with a constant velocity (assuming no air resistance). This line will be tangent to the circle that the ball/string CM was originally moving along, not tangent to the ball's trajectory. The ball continues to move along the circle because the string is moving toward the ball. These two motions are what keep the CM of the ball/string system moving in a straight. line.
@@nowandrew4442 that's a great way to look at it, yes. It's not so much the mass of the string but the rigidity of it that is the core of the unintuitive motion which makes the experimental result so interesting though. (edit - the question was where does the *ball* go, not the COM) Other than that bit you seem to agree that a rigid system if viewed as a discrete point (hence at the COM) would continue straight on once released?
This is what makes engineering the fun branch of science, sometimes life sneaks in a bit of "elasticity" to keep our minds engaged!
@ creator But then A) would be incorrect. If the COG moves **immediately** tangentially, then the tension is not the key actor. This is not the case. There is a necessary interplay between the tension wave and the COG. It can't be perfectly tangential at the moment of release. The COG *also* moves tangentially **only once the tension wave passes it** - not immediately after release. Because the COG is tethered just like the ball is. We don't see the effect of the COG release because the COG is too close to the ball, when it's light-string+ball. Use a heavier string though, and we would see the ball's movement not be tangential as the COG is already dragging it in its (the COG's) tangential path after tension wave propagation.
So the true story is: it is not the ball that awaits tension propagation, but the COG. When the tension wave reaches the COG, the COG moves tangentially. If the ball is sufficiently massive compared to the string, the COG is in the ball and those movements appear to be the same. But they are not.
Great video. This is about the level of physics I can follow (though not without some effort.) Once I accepted the premise that Person A at Point B is wearing a tie-dyed T-shirt in 2023, it was fairly easy.
Glad you touched on the sun disappearing thing at the very end, because that little fun fact about GR was the first thing that came to mind when I saw the first slinky test with the turntable. 😊
This is why I love practicing with a sling or bolas weapon. Takes a lot of skill to time your release with the swing if you want the stone to hit accurately. Also very similar to knife throwing, except knife throwing has the additional element of the knife itself wanting to spin, so to ensure the blade will hit the target, you have to actually stroke the handle of the knife as it releases to offset the knife's rotation as it travels along its arc to the target.
That particular method is often called the "no spin," or "thorn throw" technique. Takes a lot of practice to not only throw accurately, but to ensure the point of the knife makes contact as perpendicular to the surface of your target as possible. Example of this can be seen here: ruclips.net/video/oqu1hm9RKr8/видео.htmlsi=Io-K2eekpQlBUmc1
Is it not true that the slinky and rubber band are still exerting force on the ball, PULLING theball toward the center, until tension is equalized, which keeps the ball from leaving the trajectory, until all force toward the center is gone; THEN the ball continues forth as tho there were never an elastic force involved?
Yes! Right on!
Sorry, but this video is a bit misleading. He considers the elastic property of the string but not the mass. I like that he identifies the center of mass. He mentions that it is air friction that causes the slinky or ball not to be perpendicular to the tangent of the circular path the ball travels at the ball. I assure you that it is not the only (and probably not the most) cause for that phenomenon.
coriolis force applies where the orbiting object is in contact with the rotating reference frame. if a ball is attached to a string ans the string breaks, there is no such contact and the object is released tangentially. This is how a sling shot works. A satellite realease system has also been devised on this principle.
The “rotating disc with sliding puck” system is different because, although the puck is slipping, there is still a frictional force, because the puck is still in contact with the disk.
shut your ego and appreciate the vid
@@azertyuiop458 bruh thats not how education works
This is a really excellent video, well done to you sir, and the Team!!, Also really great editing and presentation.
Glad you liked it!
Vrlo kvalitetno objašnjeno.Ja sam nešto slično pokušavao pokazati sa sekirom ali nisam imao dovoljno kvalitetnu kameru da to i dokažem.Pa su me ismijavali. Ja sam naime tvrdio da ima tu nešto više od običnog udaranja i da se maksimalni efekt postiže jednom petljom a ne pravocrtnim kretanjem sekire prema drvetu.Hvala za ovaj kvalitetni video.Pozdrav iz Hrvatske.
String Theory 😂
I disagree. If you just released the ball from the string it would follow the tangent line. Since you released the ball with the string/slinky or whatever held it in tension the mass of that object and its spring forces change the motion of the ball.
But I didn’t say I was going to release the ball, I asked what would happen if I released the string! 😉
Well you sure did. I was thinking what the ball would do independently from the string the whole time but I should have gone back and listened to the question more closely.
@@wesleyashley99 Well don't feel bad, a huge number of people misinterpret the question, which is one of the reasons I think this is a fun video. They hear something different from what's being asked, and then typically yell at me and say that I'm not being fair! But I'm just making use of some well-known psychology to lure people in so I can show them something fascinating! 😉
Very interesting ! A year later I watched again and now I feel it's like the "Object in question" can be a ball or particle in which B is correct or if the "Object in question" was a three foot long ball made of rubber or a slinky which introduces many more factors that take us away from the basic principal that "Inertia wants to travel in a straight line" AWESOME THOUGHT EXPERIMENT THOUGH ! ! ! THANK YOU , it was so good I had to come back a year later to watch again ! ! !
I like the example of the centre of gravity for the slinky.....
This was an amazing demonstration, thank you! What I loved the most is how it builds intuitions about the different reference frames involved in this system to create a full understanding of their combined effect. Beautiful!
Thank you. I appreciate the kind words. Please consider sharing with others!
@@AllThingsPhysicsRUclips Will do!
I was sorta following along for most of the explaination on a
"this doesnt seem right but i trust you cause you sound like you know what youre talking about"
Basis, but with the gravity example at the very end it finally clicked as i have spent more time thinking about that sort of thing and suddenly the whole phenomenon seems intuitive!
Awesome!
superb experiments, equipment, cameras, presentation. perfect. loved it.
Thanks so much for the comment! I'm glad you enjoyed it. There will be many more videos, I promise, but it's difficult to find the time when you have a day job.
I dunno, the behaviour of the slinky & the ball on the slinky is exactly what I expected and seems totally normal to me. After all it's a slinky, not a rigid metal rod. Is that weird?
Great video. Unless I'm wrong if the release is right at the ball the tension wave delay would not happen and the ball would immediately travel on a tangent line to the circle. Correct? So answering the question correctly would depend on knowing where is the release occuring. Correct?
Reviewing he did say, "immediately after releasing the string" not immediately after releasing the ball. Still, it would be more precise to word the question such as "immediately after releasing the string at the center of rotation."
Correct on both counts!
To be fair, the problem was stated as, YOU are swinging a ball around on a string (with a video showing someone doing so). How else would you release the string except from the center? 😉
Given that the ball is not being swung by an infinitely rigid rod/object, it will take a “while” for the ball to realize that the string has been let go of.. so it does follow that the instantaneous direction of the motion of the ball right after the string is cut/released will still be along the circle… fascinating video, lovely to see the slinky experiment
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I'm doing a course on nonlinear dynamical systems. It comes at this kind of question in a different kind of way, but it's cool that I had the right intuition. I would have said (b) a month ago
What would happen to the ball's trajectory when it is held by an electro magnet to eliminate the 'slinky-effect'? My guess would be that the direction will be what classic mechanics predicts. Answer B would then be correct 'again'.
I have a question related to the ball hanging lagging behind the attachpoint of the spinning wheel. You suggested this was because of friction. But couldn't it be that the same principle that makes the ball keep spinning when let go, causes the ball to spin behind when it's accelerates in a circular motion?
Not quite. When the ball is moving around in uniform circular motion, the only forces acting on the ball are gravity, the tension force from the slinky, and air drag (there's nothing else touching the ball). Thus, air drag is the only force that could be responsible for this effect.
@@AllThingsPhysicsRUclips Thanks for your reply, I've been thinking about it and its been quite a while since my last mechanic's class.
I feel like there is more going on for explaining the behaviour of the ball. I understand the ball needs some acceleration to maintain a constant speed and it has to obtain this from the strings, so an angle is necessary. But does this explain why the ball is lagging less when the noodle is used instead of the slinky?
I understand more friction with the springy, but this doesn't explain for me the amount of lag. I think it has something to do with the tension of the string. If the string wouldnt lack behind the radius would be bigger and so a stronger centripetal force would be needed. So the string needs to be longer, The only way for the stringy to be able to be in equilibrium with the centripetal force and the radius is to start lagging behind. I have to outthink this more, but I hope you follow my reasoning. What do you think?
@@boloboyz7835 Actually, the angle is not necessary to maintain a constant speed. Once the ball is in circular motion, if there were no air drag there would be no lag angle. The reason the lag angle is so noticeable for the slinky is because the slinky is very light, and so the ball is also very light. These light objects are strongly affected by air drag.
However, it take a heavier ball to stretch the noodle to an appreciable amount, and the heavier ball is much less affected by air drag, so the lag angle is much smaller.
Does that help?
id like to ask about the time from when the pivot point end of the slinky ( or that of the elastic cord ) reaches the ball .. then the ball travels in a straight line, like in choice B, as can be seen in the slomos ..
id like to propose a different experiment, but i think yours has more practical uses .. same setup, in general, but there's a provision for ball release at the far end of the slinky and the string. i think that this would have the theoretical result that physics teachers usually discuss in class.
thanks for the video lecture, it's very enlightening. as a math teacher, i have always been in love with tangents and their real world applications, and your video is a great revelation.
Yes, releasing the ball from the string results in tangent motion instantaneously. But where’s the fun in that? 😉
This is a really interesting video. Congratulations and thanks for sharing it.
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