# Linear momentum conservation puzzle

Discussion in 'Physics & Math' started by Q-reeus, Nov 13, 2016.

1. ### danshawenValued Senior Member

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Doesn't matter. It's an answer I was looking for, even if the OP wasn't asking it.

Density arguments answer a lot of physics questions. Why should energy, linear or angular momentum, or center of mass (also intimately related to density) be any different?

It is an isolated way to treat the problem (involves nothing other than batteries, the motor, the generator, and the wires and the shaft connecting them). You did not specify the relative sizes of any of those components, not that it would matter. You didn't specify the mass, size or shape of the shaft or the angular velocity. That matters rather a lot to a question about what happens to the center of mass. I don't care what the energy capacity / density of the batteries are either, obviously, so I assume it is infinite. Nothing wrong with that. You left it open. The motor can handle any amount of current or energy, and can deliver unlimited torque and rpm to the shaft. That's still isolated, and shifts the center of mass of the system to whereever you want it to be subject to the physical extent of the shaft. Or you could have used pulleys. Same result. Whatever spins with the greatest energy density is where the energy / mass / center of mass goes.

You really had something else in mind? I doubt it is as important a consideration as energy density. Final answer.

Last edited: Dec 20, 2016

3. ### Q-reeusBannedValued Senior Member

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Dan I'm sure that all makes sense to you but not to me. Of course since energy exchange is involved in a system of some finite but unspecified dimensions, energy density will be there somehow. But 'energy density' is not an answer to the puzzle of how to some how have COM remain constant in the given notionally closed system. And the answer is properly a two-parter at that.

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5. ### danshawenValued Senior Member

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Well, fine. Use axially symmetrical cylindrical batteries and install them in the shaft between the motor and the generator. Use a commutator system for the wiring embedded in the shaft. Makes no difference if you hold the shaft and spin the motor/generator or vice versa, the COM remains constant. You didn't specify battery shapes or configurations either.

What makes COM more important than energy density, or to put it another way, why should I even want to solve the COM problem?

7. ### Q-reeusBannedValued Senior Member

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Evidently as far as you are concerned, there is no problem to solve. Best to move on then.....

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8. ### danshawenValued Senior Member

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More than that, you've gotta figure, if you hand out barometers and tell your physics class to go out and measure the height of a building with it, more than a couple are going to find unconventional ways to do it. Drop it from the top of the building and see how long it takes to hit the ground, launch it from the ground with a controlled velocity and angle until it reaches the height you want, hold it at arms length and triangulate its height, compare the ratio of its shadow on the ground to that of the barometer, & etc. These are not wrong answers either. Nor is the conventional solution necessarily optimum or takes all confounding factors into account. Use the barometer while a storm front passes and see a big error you didn't expect.

But it's a fact that unless you took energy density into account, your solution is going to get the wrong answer for the problem as specified. Specify better. The less we have to read between the lines, the better.

Last edited: Dec 20, 2016
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9. ### Q-reeusBannedValued Senior Member

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A power of folksy truths there, Dan. No doubt about it.
No. Everything needed was adequately specified, Dan. Believe it or not. The problem has been tackled from every angle - except the right one. But I do feel for your frustration.

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10. ### danshawenValued Senior Member

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No frustration here. Thanks again. You could also switch the motor off or break the wires and the center of mass will stay very much the same indefinitely. Pretty sure no one tried that approach either. Or embed the discharged battery inside of the charged one. There's no shortage of valid solutions to the problem, and I doubt you could even show your solution is unique in any important respect. Specify what cannot change, and the center of mass doesn't count because you already specified that it does. In fact, that is the only item in your short list that does change. And I have stipulated that I agree, there is no error in that particular specification. The rest of the setup can morph any way we want it to until or unless you specify what else cannot be changed. Motors and generators come in all shapes, sizes, configurations. Battery chemistry and configuration determine energy density. Vented and unvented varieties are practical. What do you mean by 'isolated' in the case of the battery? Draw us a picture before and after your solution if you wish to convince us that you got it and we failed.

If you don't think of the problem this way, you simply haven't done enough physics problems. I'm certain I haven't done enough chemistry problems, nor would I wish to.

Last edited: Dec 20, 2016
11. ### The GodValued Senior Member

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See, Q-reeus, whatever you state, the air particles are not going to give an axial kick to your shaft. Any torque or axial force must be generated by voltage and current only. So enough of dragging this nonsense, I am very confident its going to be just some silly useless stuff.

But one thing is sure, all self claimed mainstream supporter resident experts have been exposed. They are even afraid of posting anything science on any thread lest you direct them to take on your riddle.

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12. ### danshawenValued Senior Member

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Nope. Atomic batteries exist, and their function (current, voltage) are not limited by simple chemistry. They lose energy by mc^2, exactly as specified. We've established, aerodynamics, Magnus force, and impeller induced rotor winding cooling are not factors either. Some electric motors, particularly ones with commutators, add little fans at one end of the shafts too. This is not the cause of the center of mass shift, is it, Q-reeus?

Last edited: Dec 20, 2016
13. ### The GodValued Senior Member

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Q-reeus will accuse you of bringing in too many concepts. Focus man focus.

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14. ### danshawenValued Senior Member

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Sorry. My mind just doesn't work like that.

15. ### OceanBreezeRegistered Member

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This canâ€™t be the answer because angular momentum is conserved separately from linear momentum. Anyway, it is a nice gif. The axial vector of angular momentum, L

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16. ### Q-reeusBannedValued Senior Member

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I suggested somewhere otherwise?
No - it will be serious and instructive physics. No tricks.
Won't disagree with that.

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17. ### Q-reeusBannedValued Senior Member

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See my reply to The God above.

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18. ### Q-reeusBannedValued Senior Member

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19. ### danshawenValued Senior Member

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20. ### The GodValued Senior Member

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21. ### QuarkHeadRemedial Math StudentValued Senior Member

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I confess I should be sorry if this were the case generally. For myself, I would "promote and love" members who either knew what they were talking about, or came here in a genuine spirit of enquiry.
Are you a member of either category?

22. ### danshawenValued Senior Member

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Curiouser and curiouser.

The paper seems to come to the conclusion that a net movement of the center of mass of an isolated system is possible if the masses are variable (like a charged, discharged set of batteries, for example). And that such movement does not violate either Newton's 2nd or 3rd law.

Kind of agree, but using a different part of the system entirely (the part that spins). Spin it up until the energy density is high. Use that inertia to disassemble and pull the now comparatively lighter part of the system used for spinning it up through a doughnut hole void in the center. Reassemble a new spinning disk some distance further on and start spinning that one up while the old one is spun down with regenerative braking.

Doesn't really matter whether you focus on the batteries or whatever is being spun up to increase energy density. Both approaches work.

And so does letting Newton's first law of inertia and simply letting the system continue moving its center of mass at a uniform velocity in a straight line, which amounts to THE SAME PROCESS, only less jumpy.

So, are we done chasing our tails yet? I think we just demonstrated time dilation / inertia is the same process for matter, but I already understood that, even if others didn't.

Make my two liner extra crispy.

Last edited: Dec 22, 2016
23. ### Q-reeusBannedValued Senior Member

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That article was best referred to in some other thread either a new one or say here: http://www.sciforums.com/threads/thrusters-on-reactionless-spacecraft-engines.158302/
Woodward's idea is set out here: https://en.wikipedia.org/wiki/Woodward_effect#Propellantless_propulsion
Seems quite wrong to me, both in terms of his notion of how mach's principle works (advanced and retarded GW exchanges a la Sciama's original formulation), and that it focuses on capacitor internal state but seems to me neglects the rest of the energy exchange circuit. I suspect the idea violates a continuity principle but have not the time or interest to pursue it in detail.

What I will say is the answer to the #1 problem does not involve any of the ideas in the pdf article you indirectly linked to, or the Wikipedia one linked to above.

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