Spinning in a Chair Physics Demo | Calculate Increase in Angular Velocity and Kinetic Energy
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- Опубликовано: 24 апр 2023
- This is one of the most common physics demos on RUclips, but nobody takes the time to actually explain the entire problem...
Use the conservation of angular momentum to calculate the change in angular velocity as the spinning masses are pulled inward. Then calculate the change in rotational kinetic energy.
Finally we will apply some calculus to show that the work done by the centripetal force in pulling the masses inward is responsible for the change in kinetic energy.
This is truly well explained. Thank you
Glad it was helpful!
I'm dizzy just watching! 🤣
It also applies to a baseball player swinging bats of different weights and lengths.
Hmmm... You've given me something to think about. Thank you.
Sir there is a question. If we use energy conservation we get different answer than if we equate all forces. How can we solve this question by balancing forces. The question is
"A pendulum bob of mass m is suspended at rest. A constant horizontal force F = mg / 2 starts acting on it. The maximum angular deflection of the string is"?
Short Answer: As pushed from equilibrium, the pendulum is speeding up until it reaches equilibrium, then it begins to slow down. Equilibrium is not where the pendulum stops.
Long Answer: I just started working on the video for this one.
Q. Work done: Fd is the work done to overcome a force and move an object a distance d. My question is that the force of a rotating object acts towards the circle and you are bringing that object to thr centre of the circle. In which case, what force are you overcoming?
I haven't done the calcs yet but is mrw^2 actually the centrifugal force created by the rotation? I believe it is a proxy since although the centrifugal force is different to mrw^2, the differential of it is the same and that is what we use for the answer.
Do tell me if I am wrong though?
There is no outward force to 'overcome' you are providing a centripetal force inward to keep the masses moving in a circle.
If you convert the velocity in mv^2/r to angular velocity 'r*w' you get mrw^2.
@@INTEGRALPHYSICS thanks for the prompt reply. Having combined your response with more thinking it almost makes sense to me.
1 more question if I may, to clear the last hurdle:
As you rightly say in your reply, F = mv^2/r = mrw^2. However F is the force towards the centre of the circle which causes the object to rotate. Therefore this is the force already on the object before you start to bring your arms in. Why in your video have you said it is the force that you have applied to bring the object in?
Thanks in advance - and not expecting you to reply as promptly. Mindful you probably have lots of comments to reply to!
Provided the masses are pulled in slowly, the force to move the masses inward at a constant speed is equal to the centripetal force. The only places that does not hold true are at the beginning and end of the inward motion... However, I assure you, I was speeding up the entire time I pulled those masses inward (and spent the next hour motion sick).
@@INTEGRALPHYSICS I think it is only today that I have realised I don't understand work done like I thought I did:
For example, say I moved a block a distance d between time t=0 and t=1 with velocity v(t)=-4t^2 +4t.
Then the work done would be zero which I don't quite understand intuitively - I get that if I integrate F(x)dx I get 0, but unsure intuitively why - after all I sure will feel that I have done some work in my arms, having moved the block. Why did I choose my function v? I chose it so that just applying the integral definition of work done would mean the energy t=0 to 1/2 would cancel the t=1/2 to 1.
Maybe there is a video in this if you think it is interesting enough, but it might just be really basic on the other hand (I did a maths degree/masters rather than physics so may be missing some basic concepts)
You talk about getting tired in your arms. Don't confuse getting tired with doing work. Instead of your arms, imagine a spring did the work. Storing energy as it is compressed, then releasing energy as it relaxes. The net change in energy of the spring from start to finish is zero; This is what we call a conservative force, meaning the energy transfer is a reversible process.
Angular momentum is a pretend vector that seems to work for you. But, it is pretend or analogous. Angular momentum does not cause the gyroscopic effect. If that was something that always bugged you in physics like it did me, well I figured out the real cause, and it is acceleration, not momentum. Look at my video explaining it. Then mnemonic devices like laws and rules are not causal either. They are for rote memory dependent people to remember what it does NOT WHY IT DOES IT. Physics needs a reset button.
That's the point of the last 3 minutes of the video, if you fell asleep by that point nobody will blame you. Here's the mathless version of this experiment explaining the energy influx that causes the increase in rotational velocity...
ruclips.net/video/bvLgw-HWn8w/видео.html
@@INTEGRALPHYSICS You missed my point too. I am very curious about what this does to you, so let me know please. I cannot get this published, because no one wants it, but I describe a 3D rose pattern path with math, because that is the path that an atom in a disk will follow spinning and tilting. Second derivative gives acceleration normal to the spin plane (PERPENDICULAR TO SPIN). Spinning only supplies the rate for tilt velocity changes to produce acceleration, so the conservation law becomes pointless and actually wrong, because perpendicular vectors do not affect each other. For some reason, trained physicists WILL NOT see this. Can you? Angular momentum is an analogy that hides the cause of the effect from you. ruclips.net/video/Sip_9ew2RjA/видео.html
It is nice to see someone else who does not accept angular momentum as a causal explanation. I have always found the explanation for the precessing bicycle wheel inadequate. I like the way you are throwing out the book and looking at a new way to explain mechanics. Thanks for giving me something to think about. Cheers!
@@INTEGRALPHYSICS I'm very surprised. You are a rare specimen. This is a 20 year effort for me (the math is newer). Journals will not touch it. Professors from college, all but one ignore me, and even the one cannot convince the others, but he is not excited about it so much. The overwhelming lesson from this is that math cannot dictate cause. The bad news from here is that once you start second-guessing things, it grows. Gravity is the largest clue in the Universe, and the variables in gravity are probably incidental, not causal...and so on. This ruined physics and people for me. I hope you can do something with it, but it made me a quack, so watch out for that.