Conservation of Angular Momentum
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- Опубликовано: 25 июн 2024
- This video discusses the concepts of angular momentum and the conservation of angular momentum. Governing equations are provided and experiments are conducted to confirm the theories being discussed. This video is geared towards students with a basic understanding of physics concepts.
Just look how the post starts wobbling when the string is pulled in. Angular momentum is clearly transferred from the ball to the setup. Conservation of angular momentum is only conserved in an isolated system.
The demonstration of the ball on the string around 1:15 to 1:20 or so shows conservation of angular momentum only initially when the string is being pulled in through the hole in the post and the angular speed increases rapidly (as 1/r^2). Kinetic energy is added by the work done by pulling on the string. However, toward the end when he stops pulling and the string starts winding around the post, there is a torque acting on the ball due to the fact that the string is no longer applying a purely radial force. When that happens, angular momentum is no longer conserved. You can see that the string has wrapped around the post because the demonstrator unwinds it.
Here are the results that I measured:
First four revolutions 2.8 seconds. The radius of rotation, however, is NOT the length of the string. It is the distance of the mass from the vertical axis of rotation. That would be L cos(a) where L is the length of the string and a is the angle of the string to the horizontal. That angle looks to be about 30 degrees.
Last 3 revolutions: .95 seconds. Here the radius of rotation is much closer to the length of the string since the string is almost horizontal.
So the ratio of the radii is not 2:1. it is 2 cos 30:1 = 1.73:1. So, according to conservation of angular momentum, the period of rotation should be reduced by a factor of 1:(1.73)^2=.333. Comparing the periods gives a result very close to that: .95/2.8=.339 and certainly is not consistent with the conservation of angular energy. You did notice that you had to exert a force to shorten the string.....
.... where do you think the energy (that you added by doing work in pulling that string) went? It did not just disappear.
Great work.
Inspired by John Mandlbaur (who commented here as well), I'm working on a video on this topic.
John is convinced that only the first demonstration is valid and it verifies his hypothesis (because it validates his hypothesis). My video will be using your demonstration and his reaction to discuss the mentality of people who declare that science is a wrong because they have discovered some fault.
I plan to include clips of this video along with links to point people to it directly.
For full disclosure, I will agree with John that you have shown a bit of confirmation bias here. You seem to have modified the experiment only so far as it was required to match your expected result. He correctly points out that this is confirmation bias. That said, you've got my thumbs-up, and (unless you object), I may be able to direct a few more views your way. (My channel is very small.)
Just for clarity, I will be explaining why John is incorrect in his hypothesis, momentum IS conserved in these types of experiments, and I will be verifying it with an analysis of 2 different videos that show the same principle.
I can understand the concern about confirmation bias, but being that a physical experiment is being used to demonstrate a theoretical result can introduce experimental errors. In my initial demonstrations I pulled the string relatively slowly. Since the spinning mass is acting along the string and the string is rubbing over the edge of the tube at the center of rotation friction is introduced. If the string is pulled more slowly the string rubs around the edge of the tube several times over the trial period. This can produce an adverse force acting on the mass - in other words the applied force is not perfectly radial. Pulling the string more quickly tends to reduce the reaction time - but then again also increases the frictional force... Unfortunately its difficult to account for this friction. Is it negligible? I was also using a plastic pen casing which had some flex to it. Did this affect the system energy? I didn't stop my experiments at the point where it verified the theoretical result, I simply couldn't physically pull on the string any harder/faster...
In my videos I try to show that experiments are not perfect and that results can be skewed if the experiment is not set up properly. As such, I tend to show the skewed results and try to rectify the problem by improving the experiment. I see it as part of the learning process.
Feel free to reference my experiments. I will likely link to your video once it is completed.
@@labratscientific1127 Thank you. I concur with everything you said. Don't forget aerodynamic drag which scales with the square of velocity.
I've talked with John and he completely refused to listen to any of my points (such as that kinetic energy is not conserved because pulling in the string against the centrifugal force adds energy to the system). Eventually when I was showing him an error in one of his calculations he started insulting me and left.
@@15Redstones This is a common trend. He's been pedaling his same shtick for years, he does it on Quora, Twitter, Facebook, ResearchGate, anywhere he can it seems. When presented with peer reviewed studies which effectively verify angular momentum's conservation, he repeats that "there is no convincing evidence", as if simply saying that will discredit the peer reviewed studies, and demands that someone prove him wrong with a ball and string experiment as if it is God's gift to science. And we've already seen from this video what happens when someone proves him wrong with a ball and string experiment...
@@iandougherty215 I find it interesting mostly from the greater social implications. There seems to be a growing trend in people willing to outright reject the value of expertise. What does it mean in a democratic society if more and more people are willing to disregard the best available knowledge from people who have worked for years to obtain it? This isn't a good trend, and my interactions with John Mandlbaur have given me no hope for the future.
Hi, I am very impressed with the fact that you are actually taking measurements which is something that is always omitted. I do not agree with your conclusion that your experiment verifies the concept of conservation of angular momentum. Physics is not about altering the experiment to match the predictions of the theory. If you are forced to reduce the radius within a fifth of a revolution before you can achieve the prediction then surely you must realise that something is not quite right with the theory. Basically you are accelerating the ball directly toward the centre and could theoretically increase the angular velocity far beyond the prediction (in fact you exceed the prediction) but this is not due to conservation of angular momentum but simply direct acceleration. The fact that it is rotating is somewhat irrelevant. The fact is that angular momentum is not conserved. Your experiment proves this. Your initial result confirms what I have been trying to tell people for the past two and half years which is that it is not angular momentum but rotational kinetic energy which is conserved. www.baur-research.com/Physics
Thanks for your input. One of the things I'm trying to do is initiate some discussion on science and technology concepts. The experiment I conduced is a classical example and is used as an example problem in the "Fundamentals of Physics" text book by Halliday and Resnick. In the book example the authors state that this problem is indeed a conservation of angular momentum problem. I am of the belief that if I pulled the string slowly, energy was lost due to flexing of the pen tube, less than perfect tube diameter allowing for friction as the string moved around the lip of the tube, and shaking of my lab table - potentially imparting adverse external forces and torques which tends to make the experiment diverge from a true conservation of angular momentum situation. Speeding up the shift from radius 1 to radius 2 reduces the time these issues can react and corrupt the results. I don't feel I was "altering" the experiment, but improving the technique to get better results. As for "exceeding the prediction", I'm using video at 30 frames/sec, and thus there is a certain amount of error in my data reduction. At the final radius it only takes 5-6 frames for one revolution (is it 5 or 6???), and there is image smear which results makes it difficult to nail down precise timing and rotational velocity. As a result, some imperfections in the data are expected. I'm open to hearing more...
Let me make myself clear. The prediction for your experiment, if we were to conserve rotational kinetic energy as opposed to angular momentum, would be a two fold increase in angular velocity, which is exactly what your result is when you run the experiment conservatively. Your experiment is nothing less than excellent. It is a perfect result. 100%.
You get minus points for not trusting your own valid and sound findings.
The laws of physics are broken. I am sorry but it is also not my fault.
Trying do deny it by applying just enough linear force to achieve enough acceleration and graphing just how hard and fast you have pull to get enough near linear acceleration to achieve the predicted result is a little stretching of imagination.
The mistake does not lie in reality.
The maths and / or the physics is the lie.
Welcome to my world, the one in which
Angular momentum is not conserved.
@@TheAnimammal Funny...then why did results exceed those predicted for conservation of rotational kinetic energy in his later trials? Shouldn't they not have exceeded the two fold increase in angular velocity? After all, his last trial showed a four fold increase in angular velocity despite the radius change being the same over each trial. How is this possible if two fold angular velocity is the appropriate result for conservation of rotational kinetic energy?
@@iandougherty215 Because motivated reasoning is not science. Yanker.
@@TheAnimammal That is not an explanation. If conservation of angular energy was true in the context of this experiment, then it would be impossible for the ball to spin faster than the 2 fold increase predicted by angular energy's conservation that you claim. So what other physical principle is responsible for the extra speed increase? Or, should I say, how does the conservation of angular energy account for this extra speed increase?
Dear Sir, had you been trying to confirm that angular energy was conserved and were expecting a result of a two fold increase, would you have continued manipulating your experiment or would you have been very happy with the perfect result your experiment achieved on the first pull ?
How come you dont believe in the conservation of angular momentum? Having seen your website and comments around I could only see your dissatisfaction with some discrepancy in numbers in experiments which are high enough to start questioning but not high enough to completly overthrow it. And thats he reason you completly refuse it?
@@fracapolligummala3548 As part of my research and development, I was making experimental prototypes which were designed to conserve angular momentum to extremes. I discovered through experiment that angular momentum is not conserved.
Any rational person who observes an average (starts at 2 rps, radius reduced to ten percent) ball on a string demonstration will agree that it does not accelerate like a Ferrari engine as is genuinely predicted by existing physics.
This is overwhelming independent experimental proof that angular momentum is not conserved.
There is no counter evidence.
The Lab Rat has confirmed with a brilliant apparatus which achieves a perfect blind result that it is in fact angular energy that is conserved.
It is irrational not to completely refuse it.
@@TheAnimammal Cant angular energy and angular momentum be consevered at the same time? It works for linear motions.
@@fracapolligummala3548 How can the result of this experiment be two fold increase and four fold increase at the same time?
@@TheAnimammal Your right, cant be. Acctually Im adding energy when im pulling the string, dont I? So energy increases by Force times way. Since we can derive the conservation of angular momentum from linear momentum, in which I think we still believe, then the resulting discrepance in energy would be the amount we have to put in. Now that im thinking about it I remember experiments from the ISS that showed that bodys when rotating and lossing energy begin to shift their axis to the one where the masses are out the farthest from it.
It is not possible that, by the reducing the lenght of the wire (i.e. the radius), the tangential velocity increases.
Only the angular velocity increases, but there is no reason for the tangential velocity to increase, because, when you reduce the radius, there is no force that pushes the ball in a tangential direction.
If the tangential velocity increased, its kinetic energy would increase without any cause.
In other words, the work needed to stop the revolving ball doesn't change, regardless the lenght of the wire.
You agree with me then. Physics however claims that the kinetic energy increases.
@@TheAnimammal
I know and this is the reason why the current physics, in particular that concerning the gravitational force, is totally wrong.
It is absolutely possible that shortening the string accelerates the ball - if it accelerates the ball north while the ball is moving east, a quarter of a lap later this will be merged into the tangential speed of movement of the ball. The acceleration of the ball comes from the energy put in by pulling the string. In cases where you pull the string slowly, you allow more time for drag through the air to reduce the acceleration of the ball. If you did the experiment in a vacuum, all speeds of pulling in the string would lead to a four-times increase in rotation rate. The conservation of angular momentum is a misleading description of what's going on because a lot of energy is being added to the system by the pull.
@@MagicSchoolbook Wishful thinking unscientific speculation.
@@TheAnimammal You're describing your own approach - where do you think the extra energy comes from if it isn't from the pull of the string?