Experimental Proof That Energy Is Not A Conserved Quantity.

[OceanBreeze]

I’ve found that one of the beauties of physics is that quick and easy is often also good and useful. For answering the question “what is the speed of the released weights in a yo-yo despin system when the cylinder is rotating at rate X?”, the simple kinetic energy calculation I showed is all you need. It’s a much simpler system than even something like the elastic collision of 2 balls, where you must solve for both momentum and kinetic energy.

 

 

 

Sure, if that is all you wanted to know, your method is fine.

 

 

Answering a question like “what is the minimum length of the tether to change the cylinder’s rotation from X to y?” or “when should the tether be released ...?” or “at what angle is the force on the tether’s attachment point when it should be released ...?” are much harder. I’ll confess that, when I first read about a yo-yo despin system, I was surprised that it was possible to stop or reverse something’s spin this way. My intuition told me that, like a skater extending her arms, you could only slow, not stop or reverse it. I would have never thought to invent such a system.

 

I’d be awed is someone could answer either of the above questions, or describe all the systems positions, velocities, and forces as a function of time, with exact algebra and calculus. If I tried it, I’d likely chicken out, and write computer simulation to get an approximate solution – though even that would be a lot of work, as simulating a weight on a tether unwinding from a spinning cylinders is complicated.

 

 

 

In Post 23 I went through the general procedure for finding the tether length and the other rotational dynamics of the system, and again briefly in Post 49.

 

It is a fairly simple matter of matching the moment of inertia of the spacecraft with the moment of inertia of the fully extended spheres.

 

As for analyzing everything during the unwinding process, that is more difficult as it involves varying mass (actually varying moments of inertia) and I linked to a paper that has the math, in Post 2.

 

 

Moderator PS: I banned DelburtPhend, because he wasn’t follow the site rule, and in general acting trollishly. If anyone thinks I acted rashly, and would like him un-banned, please say so.

 

 

I hope you are not feeling guilty about banning that troll. You did the right thing. He was not here to have a responsive discussion, only to preach his own version of Fizzics, where “Energy is not a conserved quantity”, “Angular momentum is a useless concept in the lab” and momentum is not a vector, among many other crap statements. NASA and all the physics books have it wrong, and only he is right. You cannot have a responsive discussion with someone like that. I think banning was your only option if you want this site to be taken seriously.

[A-wal]

Moderator PS: I banned DelburtPhend, because he wasn’t follow the site rule, and in general acting trollishly. If anyone thinks I acted rashly, and would like him un-banned, please say so.

I could have sworn yesterday you said pm if you think the decision is wrong so here's the message I sent you:

 

I haven't been following the topic that closely but I don't see anything that would deserve a ban.

 

I seriously doubt his claim and think he's almost certainly overlooked something but I think that he believes it. He's not trolling.

 

Edit:

Seems to me like his only indiscretion is having the nerve to disagree.

[exchemist]

Sure, if that is all you wanted to know, your method is fine.

 

 

 

 

In Post 23 I went through the general procedure for finding the tether length and the other rotational dynamics of the system, and again briefly in Post 49.

 

It is a fairly simple matter of matching the moment of inertia of the spacecraft with the moment of inertia of the fully extended spheres.

 

As for analyzing everything during the unwinding process, that is more difficult as it involves varying mass (actually varying moments of inertia) and I linked to a paper that has the math, in Post 2.

 

 

 

I hope you are not feeling guilty about banning that troll. You did the right thing. He was not here to have a responsive discussion, only to preach his own version of Fizzics, where “Energy is not a conserved quantity”, “Angular momentum is a useless concept in the lab” and momentum is not a vector, among many other crap statements. NASA and all the physics books have it wrong, and only he is right. You cannot have a responsive discussion with someone like that. I think banning was your only option if you want this site to be taken seriously.

Seconded. I also wonder if he was 333 back again in another guise.

 

But, with the troll/nutcase safely out of the way, I am actually intrigued how to analyse correctly the setup he mentioned towards the end, in which you have a rotating wheel with the mass all at the rim, and a long string one end of which is attached to the periphery and wound onto it, and the other is attached to a static mass, so that once the slack is taken up the mass will be jerked towards the wheel.  

 

Suppose, for the sake or argument, we have a 1kg wheel of radius 0.5m, so a bit like a bicycle wheel with mass all at the rim, rotating with a tangential speed of 1m/sec, ( i.e. ω = 1/0.5 = 2/sec) and the string is attached to a 1kg mass, initially at rest w.r.t. the axis of the wheel. To keep it simple the whole lot is taken to be suspended in free space.  

 

Once the lack is taken up, my instinct is to think the jerk on the string will stop the rotation of the wheel and cause the mass to move towards the wheel at 1m/sec,  by analogy with Newton's Cradle. However is that right, and how do we conserve angular momentum in the system? 

[OceanBreeze]

 

Edit:

Seems to me like his only indiscretion is having the nerve to disagree.

 

Disagreeing with such things as conservation of energy and mathematical proofs that he is wrong is a simple indiscretion?

[OceanBreeze]

Seconded. I also wonder if he was 333 back again in another guise.

 

But, with the troll/nutcase safely out of the way, I am actually intrigued how to analyse correctly the setup he mentioned towards the end, in which you have a rotating wheel with the mass all at the rim, and a long string one end of which is attached to the periphery and wound onto it, and the other is attached to a static mass, so that once the slack is taken up the mass will be jerked towards the wheel.  

 

Suppose, for the sake or argument, we have a 1kg wheel of radius 0.5m, so a bit like a bicycle wheel with mass all at the rim, rotating with a tangential speed of 1m/sec, ( i.e. ω = 1/0.5 = 2/sec) and the string is attached to a 1kg mass, initially at rest w.r.t. the axis of the wheel. To keep it simple the whole lot is taken to be suspended in free space.  

 

Once the lack is taken up, my instinct is to think the jerk on the string will stop the rotation of the wheel and cause the mass to move towards the wheel at 1m/sec,  by analogy with Newton's Cradle. However is that right, and how do we conserve angular momentum in the system? 

 

 

I suspect the weight will not move with only translational motion (in a straight line) but will be whipped around in a slow arc, conserving angular momentum, as it must be. I will have a go at a detailed solution later.

[exchemist]

I suspect the weight will not move with only translational motion (in a straight line) but will be whipped around in a slow arc, conserving angular momentum, as it must be. I will have a go at a detailed solution later.

I don't see how it can, since the tension on the string will act directly towards the rim of the wheel, whereas moving in an arc would require a component of acceleration (due to a force) perpendicular to the direction of travel of the mass.

 

But now I think about it, when one considers the wheel and mass as a single system, with the mass is moving towards the rim of the wheel, rather than towards its CG, then the system is in fact rotating, is it not?   

[A-wal]

Disagreeing with such things as conservation of energy and mathematical proofs that he is wrong is a simple indiscretion?

I was making the point that disagreeing with something shouldn't in any way be considered an indiscretion and anyone that thinks that it is has a severe problem.

 

I don't think he was arguing with any mathematics, he was arguing with how it's being applied to real situations.

[exchemist]

I was making the point that disagreeing with something shouldn't in any way be considered an indiscretion and anyone that thinks that it is has a severe problem.

 

I don't think he was arguing with any mathematics, he was arguing with how it's being applied to real situations.

Well he was arguing with Noether's Theorem, surely?  

[A-wal]

Well he was arguing with Noether's Theorem, surely?  

:shocked: Let's find him and stone him to death! A physical theory is a description of the mathematics that apply to real situations.

[exchemist]

 

:shocked: Let's find him and stone him to death! A physical theory is a description of the mathematics that apply to real situations.

 

Yes I was being a bit facetious. But on balance I agree with the ban, as this person shows no signs of being susceptible to argument and is taking a position that strikes me as absurdly dismissive of science. 

[A-wal]

Then why the hell hasn't xyz been banned a hundred (not really an exaggeration) times?

 

He's dismissive of what he believes is wrong science. Is it really against the rules of the forum to voice a different opinion?

[DrKrettin]

Is it really against the rules of the forum to voice a different opinion?

 

There would be no point in having a forum if differences of opinion were not allowed, but it rather depends on what the opinion is about. If someone voices the opinion that 1 + 1 = 3 and then generates a thread of 5 pages arguing that everybody else in the world has got it wrong, then either he/she is of impenetrable stupidity or is trolling. Neither option is particularly beneficial to a forum.

[exchemist]

Then why the hell hasn't xyz been banned a hundred (not really an exaggeration) times?

 

He's dismissive of what he believes is wrong science. Is it really against the rules of the forum to voice a different opinion?

You have a good point about xyz. 

[OceanBreeze]

 

But now I think about it, when one considers the wheel and mass as a single system, with the mass is moving towards the rim of the wheel, rather than towards its CG, then the system is in fact rotating, is it not?   

 

 

Yes. The attached mass will not be reeled in by the wheel at all. If it is X meters from the point of attachment on the rim, when the line goes slack, it will remain X meters away at all times. What will happen is the system consisting of wheel and attached mass will slowly rotate about the com of the system, in the same direction the wheel was rotating before, conserving angular momentum and rotational kinetic energy.

This has to be true because the com of this isolated system cannot move due to internal forces. (Otherwise this would be some sort of reactionless drive and that cannot be).

One key factor we do not have is how long the string is when the line goes taut. That is needed in order to work out the moment of inertia of the system. That moment of inertia will be a bit complicated. I believe the parallel axis theorem will be needed.

[exchemist]

Yes. The attached mass will not be reeled in by the wheel at all. If it is X meters from the point of attachment on the rim, when the line goes slack, it will remain X meters away at all times. What will happen is the system consisting of wheel and attached mass will slowly rotate about the com of the system, in the same direction the wheel was rotating before, conserving angular momentum and rotational kinetic energy.

This has to be true because the com of this isolated system cannot move due to internal forces. (Otherwise this would be some sort of reactionless drive and that cannot be).

One key factor we do not have is how long the string is when the line goes taut. That is needed in order to work out the moment of inertia of the system. That moment of inertia will be a bit complicated. I believe the parallel axis theorem will be needed.

Not sure about that. But I realised later on that if one considers the CG of the system (the mass and the wheel), then once the line goes taut and jerks the mass towards to wheel, the mass distribution, consisting as it does of 2 parts, the mass and the wheel, is then rotating about the common CG of the system as the mass and wheel converge, because the mass is not aligned with the CG of the wheel. 

 

Which just goes to show how careful one has to be in attempting to analyse such systems. And which is again doubtless why our friend liked them so much as - given a sufficiently inflated ego - he could easily convince himself he was right and science was wrong. :)

Edited January 5, 2017 by exchemist

[DelburtPhend]

Thanks A-wal: I shall try to be super polite;

 

If you fire a high speed bullet (tangent) into the circumference of a flywheel: which one of the three formulas (angular momentum, linear momentum, kinetic energy) are you going to use. This experiment can be done (or looked up) so we will need a formula that will work. There is only one choice.

 

This bullet into the flywheel is a variation of the ballistics pendulum. Instead of the pendulum bob mass moving: the entire bearing-ed rim mass will spin together as one unit. Massive amounts of motion energy will be lost: and the bullet starts with linear motion. There is no initial angular momentum so that can not be conserved. Linear Newtonian momentum conservation tells us how much motion energy is lost.  Linear Newtonian momentum is the only formula that can tell us the arc motion of the wheel. Angular momentum conservation and energy conservation cannot tell us the motion of the wheel.

 

This might help you see that this is simpler than it looks. Let the bullet be fired into two 1 kilogram masses that are separated by hard Styrofoam. After the bullet gives them a velocity; the 1 kilogram masses are captured in a circular path at 180° across the circle. The velocity of these two 1 kilogram masses will be the same as a 2 kg flywheel.

 

Okay: let keep in mind that Linear Newtonian Momentum is the only formula that works?

 

Now lets go back to the Dawn Mission; if the Dawn Mission de-spin event had not released the tethers, at extension, all the motion would have been returned to the satellite. This is one of the concepts proven by the cylinder and spheres experiments; all the motion is returned to the cylinder even after it was stopped twice.

 

So how does the Dawn Mission re-spin (return of all rotational motion) differ from the bullet into the 2 kilograms.

 

Now I see different people trying their hand at evaluating the Dawn Mission; that's great. But one thing I did not see is an actual velocity given for the 3 kilograms at extension. Just from memory I think the KE conservation velocity number would be 20 m/sec. And I think the angular momentum conservation velocity would be about 16 m/sec. Newtonian momentum conservation would put the number at 400 m/sec; just looking at the videos of such events which do you think is more likely?

 

What information can the cylinder and spheres experiment give us about the Dawn Mission. One piece of information has already been stated; All the motion is returned to the stopped cylinder; and that would be true of the Dawn Mission as well.

 

The bullet into the 2 kilogram: and the spheres into the cylinder are very much alike; and what is the only formula that tells us the rotational motion of the cylinder?

 

A friend of mine made a slow motion video of the double stop cylinder and spheres; I would be more than happy to send any or all of you (posting) a flash drive copy.

 

You could probably talk me into sending an actual model.

 

Thanks for not booting me; I think the double stop cylinder and spheres (and similar experiments) is an important contribution to science, and if I am booted all is lost.

[DelburtPhend]

The purple arrow is on the monitor. The camera is on a tripod and it did not move; these are two consecutive frames from a 240 frame/sec camera. This is the first stop.