A simple classical physics / algebra question

[Mandlbaur]

I'd like to see your analysis of this demonstration, which is significantly better than the sling thing, since it actually approaches an isolated system. How would you calculate the speed after the professer pulls the weights to his chest? The body of the professor clearly speeds up, yet its radius remains constant.

 

 

Please see Walter Lewin's Lecture, the demonstration takes place at around 24:00 his angular velocity increases but not as much as his calculation claims.

[Bender]

So what are the angular velocities in the demonstration? When his arms are stretched, it takes about 2 s to take half a turn; when his arms are to his chest, it takes less then 1 s (I honestly can't measure it more precise than that from the video). His very rough estimates predict a tripling of the angular velocity, so in the second situation that would be about 0.7 s, which is spot on, given the accuracy of the demonstration and the accompanying estimates (which is already orders of magnitude more accurate than your demonstrations).

 

What does your calculation claim? I am genuinely interested in how you would calculate/estimate this without conservation of angular momentum.

 

EDIT: you never answered my question about how you feel about conservation of linear momentum and Newton's laws of physics. If you still refuse to, I'll have to assume you are either trolling or that you want to avoid having to rethink your misconceptions.

Edited March 15, 2017 by Bender

[Mandlbaur]

So what are the angular velocities in the demonstration? When his arms are stretched, it takes about 2 s to take half a turn; when his arms are to his chest, it takes less then 1 s (I honestly can't measure it more precise than that from the video). His very rough estimates predict a tripling of the angular velocity, so in the second situation that would be about 0.7 s, which is spot on, given the accuracy of the demonstration and the accompanying estimates (which is already orders of magnitude more accurate than your demonstrations).

 

What does your calculation claim? I am genuinely interested in how you would calculate/estimate this without conservation of angular momentum.

 

EDIT: you never answered my question about how you feel about conservation of linear momentum and Newton's laws of physics. If you still refuse to, I'll have to assume you are either trolling or that you want to avoid having to rethink your misconceptions.

 

 

Using a stopwatch, I measured the time for the full arms extended rotation between 24:35 and 24:39 from the point where the green weight aligns with his shoulder. My average of three closely matched measurements is a little under 3.6 seconds. I also measured the time for the full arms retracted rotation between 24:52 and 24:54 from the point where both weights align. The average of seven measurements is a little more than 1.7 seconds. The expected time as per the calculations is 1.2 seconds so there is a difference of at least 30% and it indicates a significant reduction in angular momentum.

 

I will get back to you once I have figured out the maths without conserving angular momentum.

 

I refuse to answer your question because it is irrelevant to the discussion and I believe it is nothing more than an attempt to find some reason to discredit me.

 

I will say this though: in the equation L=r x p, when we change the magnitude of r it is not possible to conserve both L and p. (assuming of course that p is not zero and not parallel to r). My money is on linear momentum being conserved (in terms of it's magnitude).

[Argent]

I refuse to answer your question because it is irrelevant to the discussion and I believe it is nothing more than an attempt to find some reason to discredit me.

It seems a reasonable and relevant question to me. Indeed, it seems to strike straight to the heart of the matter.

 

I see no evidence that Bender is trying to discredit you, but he is trying to discredit your idea. That is entirely proper. It is the scientific approach. Any claims should be challenged, especially when they conflict with established science. He is providing you with the opportunity to substantiate your claim. If you fail to do so my conclusion, and I think the conclusion of most others, will be that your claim is empty.

[Mandlbaur]

The question was: "How do you feel about conservation of linear momentum and Newton's laws of physics?"

 

Please explain how that is relevant to this discussion and how it can possibly substantiate anything?

 

Let me give you my answer: I feel good!

[swansont]

 

I will say this though: in the equation L=r x p, when we change the magnitude of r it is not possible to conserve both L and p. (assuming of course that p is not zero and not parallel to r). My money is on linear momentum being conserved (in terms of it's magnitude).

 

 

The linear momentum of the system is zero and remains zero, so yeah, it's conserved.

[Bender]

Of course, I am not trying to discredit you. Apparently, you have no problem with conservation of linear momentum. If, on top of that, you agree with Newton's laws of motion, your problem with conservation of angular momentum is nonsensical.
Angular momentum can be seen as a mathematical construct derived using nothing more than Newton's laws of motion and rigorous mathematics. Denying one means denying the other; acknowledging one means acknowledging the other.

Using a stopwatch, I measured the time for the full arms extended rotation between 24:35 and 24:39 from the point where the green weight aligns with his shoulder. My average of three closely matched measurements is a little under 3.6 seconds. I also measured the time for the full arms retracted rotation between 24:52 and 24:54 from the point where both weights align. The average of seven measurements is a little more than 1.7 seconds. The expected time as per the calculations is 1.2 seconds so there is a difference of at least 30% and it indicates a significant reduction in angular momentum.

As I said: spot on. There is easily 10-20% error on your measurement. The expected time as per the estimations is not "1.2 seconds", but "somewhere about 1.2 seconds", especially since he probably just approximated his body as a uniform cylinder, which is obviously a pretty large simplification.

I will get back to you once I have figured out the maths without conserving angular momentum.

I refuse to answer your question because it is irrelevant to the discussion and I believe it is nothing more than an attempt to find some reason to discredit me.

I will say this though: in the equation L=r x p, when we change the magnitude of r it is not possible to conserve both L and p. (assuming of course that p is not zero and not parallel to r). My money is on linear momentum being conserved (in terms of it's magnitude).

Obviously, both are conserved . Of course, only the total linear momentum is conserved, and the total linear momentum is zero throughout the entire experiment, since the centre of mass remains stationary.

 

EDIT: the linear momentum you refer to in the equation is the linear momentum of individual small parts of the professor and the weights. That is not conserved, since there are forces acting between all the individual parts. Adding all the linear momentum of all the parts however, keeps adding up to zero.

Edited March 15, 2017 by Bender

[Mandlbaur]

Of course, I am not trying to discredit you. Apparently, you have no problem with conservation of linear momentum. If, on top of that, you agree with Newton's laws of motion, your problem with conservation of angular momentum is nonsensical.

Angular momentum can be seen as a mathematical construct derived using nothing more than Newton's laws of motion and rigorous mathematics. Denying one means denying the other; acknowledging one means acknowledging the other.

 

As I said: spot on. There is easily 10-20% error on your measurement. The expected time as per the estimations is not "1.2 seconds", but "somewhere about 1.2 seconds", especially since he probably just approximated his body as a uniform cylinder, which is obviously a pretty large simplification.

 

Obviously, both are conserved . Of course, only the total linear momentum is conserved, and the total linear momentum is zero throughout the entire experiment, since the centre of mass remains stationary.

 

EDIT: the linear momentum you refer to in the equation is the linear momentum of individual small parts of the professor and the weights. That is not conserved, since there are forces acting between all the individual parts. Adding all the linear momentum of all the parts however, keeps adding up to zero.

 

 

I believe that there must be errors within the derivations because they defy the simple logic contained within my OP which has yet to be successfully challenged.

 

I will grant you a generous error of +-20% on the final discrepancy. This would leave us with a discrepancy of between 10% and 50%. This can not in any way especially scientifically be described as "spot on".

[Bender]

I believe that there must be errors within the derivations because they defy the simple logic contained within my OP which has yet to be successfully challenged.

You are free to believe that, but that doesn't make it true. There is also no contradiction with the logic in the OP, since, as has been carefully explained by different members, it doesn't apply.

 

I will grant you a generous error of +-20% on the final discrepancy. This would leave us with a discrepancy of between 10% and 50%. This can not in any way especially scientifically be described as "spot on".

Ok, I'll rephrase it more scientifically: the predicted time was [math]1.2 \pm 0.5 s[/math] and the measured time is [math]1.7 \pm 0.3 s[/math]. Since the error ranges overlap quite a bit, there is no significant difference and the zero hypothesis holds. Edited March 15, 2017 by Bender

[Mandlbaur]

You are free to believe that, but that doesn't make it true. There is also no contradiction with the logic in the OP, since, as has been carefully explained by different members, it doesn't apply.

 

Ok, I'll rephrase it more scientifically: the predicted time was [math]1.2 \pm 0.5 s[/math] and the measured time is [math]1.7 \pm 0.3 s[/math]. Since the error ranges overlap quite a bit, there is no significant difference and the zero hypothesis holds.

 

 

Every careful explanation provided has been effectively countered.

 

Sure if you randomly select a sufficiently inaccurate margin of error. However it seems to me that an error margin of +-0.2 seconds is more than sufficient to account for human reaction times and if I look at the deviations in my actual measurements, the accuracy for the longer measurement is +-0.01 and for the shorter measurement is +-0.07. That is why I described the error margin I offered as generous. The difference is significant.

[Bender]

Never mind your error. If prof. Lewin had bothered, he could easily have modified the estimates he used to fit the measurements exactly, since the result is quite sensitive to deviations.

 

He didn't bother, because that was not the purpose of the demonstration.

 

I notice how you still persist in ignoring the issue that angular momentum is nothing but an application of Newton's laws of motion.

Edited March 16, 2017 by Bender

[Phi for All]

Every careful explanation provided has been effectively countered.

 

!

Moderator Note

No.

 

Every careful explanation provided to you has been effectively ignored. It's clear you have a mental logjam about this, and can't see your way out of it. The "logic" in your OP you keep referring to seems to be keeping your head below water. I suggest you re-examine. This thread can't continue in Speculations unless you can support the explanation with something more than waving hands.

 

Last chance. Start listening, or start providing supportive evidence for your stance.

[Strange]

I notice how you still persist in ignoring the issue that angular momentum is nothing but an application of Newton's laws of motion.

 

 

And also, if you do an actual experiment, with measurements, rather than trying to analyse yootoob videos, you will find that angular momentum is conserved. This is the sort of thing that is done in physics labs at school and university thousands of times a year. If there were really a problem, I think someone would have noticed by now.

 

Your reliance on bogus "logic" and yootoob (and avoidance of real data) totally undermines your case.

Edited March 16, 2017 by Strange

[Mandlbaur]

Never mind your error. If prof. Lewin had bothered, he could easily have modified the estimates he used to fit the measurements exactly, since the result is quite sensitive to deviations.

 

He didn't bother, because that was not the purpose of the demonstration.

 

I notice how you still persist in ignoring the issue that angular momentum is nothing but an application of Newton's laws of motion.

 

 

Please provide some evidence to support your claim?

 

Please explain the purpose of this demonstration?

 

My argument that conservation of angular momentum defies Newton's laws can be found here: http://www.baur-research.com/Physics/CAMFI.pdf

 

 

And also, if you do an actual experiment, with measurements, rather than trying to analyse yootoob videos, you will find that angular momentum is conserved. This is the sort of thing that is done in physics labs at school and university thousands of times a year. If there were really a problem, I think someone would have noticed by now.

 

Your reliance on bogus "logic" and yootoob (and avoidance of real data) totally undermines your case.

 

 

Please provide a reference to an experiment to which supports your claim? I have been unable to find one. An argument that since it has not been found for a long time period in history means that there isn't a problem is not very scientific.

 

!

Moderator Note

No.

 

Every careful explanation provided to you has been effectively ignored. It's clear you have a mental logjam about this, and can't see your way out of it. The "logic" in your OP you keep referring to seems to be keeping your head below water. I suggest you re-examine. This thread can't continue in Speculations unless you can support the explanation with something more than waving hands.

 

Last chance. Start listening, or start providing supportive evidence for your stance.

 

 

As far as I am aware, I have countered every reasonable argument levelled against my OP. Could you please point out any argument that I have failed to address?

 

I have provided calculations which show that the ball on a string demonstration does not produce expected results and I have provided verifiable measurements which show that the hand weighted turntable professor demonstration does not produce expected results. I believe that this covers the vast majority of evidence provided to students when they are being taught about the concept of conservation of angular momentum.

 

Surely this must be seen as effort on my part toward providing supporting evidence.

 

If I have made any questionable claim for which I have not provided supporting evidence, please point it out and I will try to oblige.

[Strange]

Please provide a reference to an experiment to which supports your claim? I have been unable to find one.

 

 

Sorry, you don't get to shift the burden of proof. You are making a claim, it is up to you to support it.

 

I did say, "if you do an actual experiment..."

 

So, if you don't think anyone else has ever measured angular momentum, then go ahead and do it yourself. But do it properly. Stop messing about. Generate some real data to support your case. Submit it to a scientific journal. Get a Nobel Prize. It can't be too hard if the deviation from expected results are as enormous as you claim.

 

Doing an idiotic caricature of science with yootoob videos gives you about as much credibility as an Apollo hoax believer.

 

 

As far as I am aware, I have countered every reasonable argument levelled against my OP.

 

Sadly, this is what people like you always say. You have failed to address any arguments against your idea.

 

 

If I have made any questionable claim for which I have not provided supporting evidence, please point it out and I will try to oblige.

 

You claim that angular momentum is not conserved and have provided zero evidence for that.

Edited March 17, 2017 by Strange

[Mandlbaur]

 

 

Sorry, you don't get to shift the burden of proof. You are making a claim, it is up to you to support it.

 

I did say, "if you do an actual experiment..."

 

So, if you don't think anyone else has ever measured angular momentum, then go ahead and do it yourself. But do it properly. Stop messing about. Generate some real data to support your case. Submit it to a scientific journal. Get a Nobel Prize. It can't be two hard if the deviation from expected results are as enormous as you claim.

 

Doing an idiotic caricature of science with yootoob videos gives you about as much credibility as an Apollo hoax believer.

 

Sadly, this is what people like you always say. You have failed to address any arguments against your idea.

 

 

You claim that angular momentum is not conserved and have provided zero evidence for that.

 

 

 

I have provided a logical deduction based on valid premises - this is the definition of proof. Therefore I have fulfilled the requirements of "burden of proof".

 

The latest version of my proof can be seen here: http://www.baur-research.com/Physics/CAMFI2e.pdf

 

Since you have made unsupported claims against my work, it is perfectly reasonable for me to ask you to substantiate them. The burden of proof for your claims lies with you.

 

I have submitted my work many times in many different versions to many different journals and have yet to face peer review - it is rejected by the editor immediately probably because it "does not agree with generally accepted principles". It is not possible for me to produce a paper showing that there is a flaw in the generally accepted principles which also agrees with the generally accepted principles. Which is why I have come here.

 

Please point out any specific argument which I have failed to address - I sincerely believe that I have addressed all of them. I reiterate: the burden of proof for your claims lies with you.

 

I have provided a logical proof of my claim. Proof is the highest quality of evidence that can be provided. Unless you can show my premises to be invalid or my logic to be flawed, you have to accept a conclusion drawn by this process.

 

Please refrain from insult it is nothing more than ad-hominem.

 

I would also like to suggest that perhaps it would be wise to consider, since you are obviously extremely emotionally charged, the possibility that your reasoning might not be quite as rigorous and reliable as is usual.

[Strange]

I have submitted my work many times in many different versions to many different journals and have yet to face peer review - it is rejected by the editor immediately probably because it "does not agree with generally accepted principles". It is not possible for me to produce a paper showing that there is a flaw in the generally accepted principles which also agrees with the generally accepted principles. Which is why I have come here.

 

 

Perhaps you could show here some of the experimental data that you put in these submissions. You do have experimental data, don't you? If not, that may be why it was rejected, rather than some sort of blind prejudice.

 

After all, science progresses by challenging and occasionally overthrowing generally accepted principles. That is what the big prizes are awarded for.

 

 

 

Please point out any specific argument which I have failed to address

 

You have provided no evidence.

 

 

 

I reiterate: the burden of proof for your claims lies with you.

 

Wrong. I make no claim. I am asking you to provide evidence to support yours.

 

 

Please refrain from insult it is nothing more than ad-hominem.

 

What insult?

 

 

 

I would also like to suggest that perhaps it would be wise to consider, since you are obviously extremely emotionally charged, the possibility that your reasoning might not be quite as rigorous and reliable as is usual.

 

Why do you think I am "emotionally charged"? I find your stubborn refusal to provide evidence, or even to see that it is required, mildly amusing. But that is all.

 

Why is it that people with their own personal theories are convinced that they make others angry, or scared or emotional. I don't get it. But ho, hum.

Edited March 17, 2017 by Strange

[swansont]

Excerpt from your link (you couldn't just post this?)

 

"If we apply classical newtonian mechanics to this scenario then we have to admit that the only relevant force we are ap- plying to the object is through the string which is perpendicular to the object’s rotational component of velocity. In order to change the magnitude of the rotational component of the velocity of the object, we have to apply a force which has a component that is in the same direction as that velocity component (Newton’s First Law). As there is no component of any perpendicular force that is in the same direction as the respective velocity, we cannot possibly influence the magni- tude of the rotational component of the velocity of the mass in the example "

 

If we reduce R to 1/2 of its value, conservation of angular momentum predicts that the angular speed (w) will double. You claim that in the absence of a tangential force, Newton's first law says this can't happen. But that applies to linear motion, i.e. v, rather than w. And if you halve the radius while keeping v constant (as you properly claim it will), w must double.

 

Congratulations. Your argument shows that angular momentum will be conserved.

[Mandlbaur]

Excerpt from your link (you couldn't just post this?)

 

"If we apply classical newtonian mechanics to this scenario then we have to admit that the only relevant force we are ap- plying to the object is through the string which is perpendicular to the object’s rotational component of velocity. In order to change the magnitude of the rotational component of the velocity of the object, we have to apply a force which has a component that is in the same direction as that velocity component (Newton’s First Law). As there is no component of any perpendicular force that is in the same direction as the respective velocity, we cannot possibly influence the magni- tude of the rotational component of the velocity of the mass in the example "

 

If we reduce R to 1/2 of its value, conservation of angular momentum predicts that the angular speed (w) will double. You claim that in the absence of a tangential force, Newton's first law says this can't happen. But that applies to linear motion, i.e. v, rather than w. And if you halve the radius while keeping v constant (as you properly claim it will), w must double.

 

Congratulations. Your argument shows that angular momentum will be conserved.

 

 

Conservation of angular momentum predicts that if we halve the radius, the angular speed will quadruple.

[swansont]

 

 

Conservation of angular momentum predicts that if we halve the radius, the angular speed will quadruple.

 

 

 

Sorry, yes that's true. Pre-caffeine posting is dangerous.

 

There's more to the story.

 

When an object moves in a circle, the force must be to the center, and no tangential force. But while the radius is being halved, the motion is not circular and the force is not acting toward the center of the circle. There is indeed a tangential force.

[Bender]

"If we apply classical newtonian mechanics to this scenario then we have to admit that the only relevant force we are ap- plying to the object is through the string which is perpendicular to the object’s rotational component of velocity. In order to change the magnitude of the rotational component of the velocity of the object, we have to apply a force which has a component that is in the same direction as that velocity component (Newton’s First Law). As there is no component of any perpendicular force that is in the same direction as the respective velocity, we cannot possibly influence the magni- tude of the rotational component of the velocity of the mass in the example "

If the radius is reduced, the angular velocity of the outside mass will increase, and it will fly past the perpendicular position of where the string is attached to the central mass, resulting in a tangential component of the force in the string, which slows down the outside mass and causes a torque on the central mass.

[Mandlbaur]

 

 

 

Sorry, yes that's true. Pre-caffeine posting is dangerous.

 

There's more to the story.

 

When an object moves in a circle, the force must be to the center, and no tangential force. But while the radius is being halved, the motion is not circular and the force is not acting toward the center of the circle. There is indeed a tangential force.

 

 

The force is always acting to towards the centre of rotation. There can be no perpendicular component of the applied force. Therefore there can be no affect to the component of the velocity that affects the angular velocity.

 

You have already agreed with this.

 

You agree with my work provided that you think it supports your argument. Can you not see how unreasonable your position is?

Edited March 17, 2017 by Mandlbaur

[Bender]

The force is always acting to towards the centre of rotation. There can be no perpendicular component of the applied force. Therefore there can be no affect to the component of the velocity that affects the angular velocity.

Why can there be no tangential component? Please keep the discussion to isolated systems, such as the rotating professor, and not the mass on a string, which isn't isolated.

[swansont]

The force is always acting to towards the centre of rotation. There can be no perpendicular component of the applied force. Therefore there can be no affect to the component of the velocity that affects the angular velocity.

Let's turn this around. If the force is always toward the center of the circle, there can be no work done and the object must continue moving in a circle. Centripetal force and circular motion are intimately tied together.

 

But pulling on the string is doing work on the system, and the path is not circular. There is no way to have that be true without a tangential component.

 

You have already agreed with this.

I was addressing the fact that the angular velocity can change without there being a force. I also admitted that part of my claim was in error, and was correcting that.

 

If you're going to appeal to "no takebacks" then you can go back to elementary school.

 

You agree with my work provided that you think it supports your argument. Can you not see how unreasonable your position is?

I will agree with what is in accord with physics. There is nothing unreasonable about that.

[Mandlbaur]

 

Let's turn this around. If the force is always toward the center of the circle, there can be no work done and the object must continue moving in a circle. Centripetal force and circular motion are intimately tied together.

 

But pulling on the string is doing work on the system, and the path is not circular. There is no way to have that be true without a tangential component.

 

 

I was addressing the fact that the angular velocity can change without there being a force. I also admitted that part of my claim was in error, and was correcting that.

 

If you're going to appeal to "no takebacks" then you can go back to elementary school.

 

 

I will agree with what is in accord with physics. There is nothing unreasonable about that.

 

 

There is always a force on the string whether the radius is increasing, decreasing or the force is balanced and the radius remains constant. (I am aware that I have Omitted certain required provisos, but I am sure we are all intelligent enough to understand that these are implied and we can keep the discussion within the bounds intended). There is always work being done. If anybody has noticed, I have corrected my error and replaced tangential with perpendicular. There cannot be a perpendicular component of the force. Angular velocity can change without there being a force because it changes with the radius, yes, but the component of velocity which affects angular velocity cannot. i.e.: The component of velocity which is perpendicular to the radius cannot be affected without a perpendicular component of force.

 

You agree with an argument that you think supports your position and then disagree when it is pointed out that it does not support your position. This clearly indicates that it is your position which stands and not the validity of the argument. A strong indication that you are subject to confirmation bias.

Edited March 17, 2017 by Mandlbaur