# A Problem

Discussion in 'Physics & Math' started by hansda, Nov 9, 2016.

1. ### river

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If you stopped the Earth spinning , the kinetic energy from the spin will cause tectonic plates to move , water will cause enormous waves , the Earths atmosphere will do bizzare things as well , but the kinetic energy produced by stopping the Earths rotation , will subside , in time .

The energy , the kinetic energy , the angular momentum , consequence , will disapate, into energy of zero kinetic energy .

3. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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Really? If an someone on the space station were to grab a spinning ball the angular momentum would be transfered to the person. So the person would begin to slowly spin. The ball would have stopped spinning relative to the person but not relative to the space station. If the person was attached to the space station then the angular momentum would be transfered to the space station.

Due to the inelastic nature of people some of the angular momentum would cause a deflection of the skin, muscles and bones so the final angular momentum would actually be less than the starting angular momentum and the 'missing' angular momentum would be lost in the form of heat (though friction).

This is all very logical and straight forward if you realize that angular momentum is conservered.

5. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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This is all because angular momentum is conserved.

7. ### SarkusHippomonstrosesquippedalo phobeValued Senior Member

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And by looking just at the earth you are looking at one element of the system - not the system as a whole.

8. ### river

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Sure

But how does this change anything I have said

9. ### river

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Is it though ?

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Yes.

11. ### SarkusHippomonstrosesquippedalo phobeValued Senior Member

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Because in post #16 you said you disagreed with my explanation of how the angular momentum within a system is conserved, yet your subsequent explanation for why you disagree only looks at one element of the system and NOT the system as a whole!

12. ### Russ_WattersNot a Trump supporter...Valued Senior Member

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You can apply the OP's scenario to three real-world objects that function exactly as the scenario describes:
The Observer is the Hubble Space Telescope.
"Ball 1" is Jupiter.
"Ball 2" is a flywheel onboard the Hubble Space Telescope, but magnetically isolated (normally not touching or otherwise interacting).

The Hubble Space Telescope does indeed use the slowing down and speeding up of flywheels to point itself and in so doing has no impact on Jupiter.

13. ### hansdaValued Senior Member

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Correct. But is this system isolated? Isn't the Earth also playing some role in this system?

Think of an isolated solar system. If somehow the spin of one planet is stopped in this solar system; Will the spin of other planets in the solar system change or not for conservation of angular momentum of the solar system?

14. ### hansdaValued Senior Member

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Is it? Suppose in a football field the goalkeeper holds a spinning ball and stops it; Will the goalkeeper along-with the ball, spin at a much slower rate?

You mean to say, if the observer stands in a house, the entire house will tend to rotate?

15. ### exchemistValued Senior Member

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What is wrong with you? Can you not see that "if the spin of a planet is stopped", then whatever it is that does the stopping will acquire the angular momentum lost by the planet?

Just as was pointed out to you about your "observer" in the OP, which was not just an observer but a part of the system, because he/she/it interacted with other elements of the system.

So for your new scenario to have meaning, you can't just say suppose the spin of a planet were stopped "somehow": you have to specify how. And as soon as you do that, the answer to what happens to the angular momentum will be obvious. So, how do you propose the spin of this planet would be stopped? By interaction with what? Because, you see, it won't just happen by magic.

Russ_Watters and sideshowbob like this.
16. ### exchemistValued Senior Member

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Yes. In these cases, because the goalkeeper is attached to the Earth, the angular momentum goes into the Earth-goalkeeper assembly, which is so vast of course that no change is perceptible. (In a similar way, when a sprinter accelerates, in a 100m race, he or she exerts a force on the ground that alters the rotation of the Earth, although of course to a negligible degree.) But if you had a goalkeeper in free fall in a space station, i.e. not part of a vastly larger system, then most certainly you would see him or her start to rotate slowly.

17. ### Russ_WattersNot a Trump supporter...Valued Senior Member

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In terms of the system's angular momentum, it is isolated and the Earth plays no role. The telescope is balanced so that tidal forces from Earth do not alter its orientation/make it spin.
The spin of the other planets will not change.

You are trying to find an inconsistency in conservation of momentum by violating conservation of momentum. You're working under the belief that conservation of momentum for the system should be conserved (it should be) and then by *magic* you remove some momentum and are claiming that's an inconsistency in conservation of momentum (or a spooky action at a distance to resolve it). No, the only inconsistency is your invoking of magic.

Last edited: Nov 15, 2016
18. ### hansdaValued Senior Member

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May be you are correct but dont know if this is experimentally proven.

19. ### Russ_WattersNot a Trump supporter...Valued Senior Member

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Of course it is experimentally proven! We've been doing work in space for more than 60 years and what you describe is a normal part of interactions between objects!

20. ### exchemistValued Senior Member

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If you watch any video of people in the ISS you can see this sort of thing. And it is manifest in other situations, on Earth, too. If you have a swimmer rotating and someone grabs her she will slow down and he will start to rotate. If a child on roller skates tries to stop a playground roundabout, he slows it down but also starts to rotate. There is nothing remotely uncertain or controversial about any of this. It has been part of standard mechanics for a couple of hundred years at least. Any interaction with a rotating object is liable to transfer angular momentum to whatever it is that interacts. But with no interaction, there is no transfer.

It gets more complicated when there are less obvious types of interaction, such as tidal effects between astronomical bodies, but the principles remain just the same. If in your scenario the planets are too far apart and/or rigid for such effects to be significant, then there will not be any change in angular momentum of any of them.

P.S. Have a look at the girl holding the spinning bicycle wheel in this (very short but rather nice) video:
She is on a turntable with a vertical axis, holding a bicycle wheel with its axis of rotation horizontal. Someone gets the wheel spinning about this horizontal axis. She is stationary. Then she turns it so that the spin axis is vertical, whereupon she starts to rotate in the opposite sense, preserving zero net angular momentum about the vertical axis. She then turns it over so it is again spinning about a vertical axis in the other direction...and she rotates in the opposite direction from how she did the first time, again maintaining zero net angular momentum about the vertical axis.

Of course, if she were just standing on the floor she would not rotate visibly. She would just feel a twisting force on her body, through to her feet, but this would in effect fractionally alter the rotation of the Earth. The free-running turntable, however, makes the effect visible by isolating her from the friction with the ground.

Last edited: Nov 15, 2016
21. ### exchemistValued Senior Member

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Seems my link to the video has disappeared, for some reason. Here it is again:

22. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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Yes the angular momentum will be transfered to the person and the house. And if the house is on earth then the angular momentum will be transfered to the person/house/earth. Do I think the house will rotate? N0, but the momentum is still transfered. If a large truck is driving down the road and it hits a mosquito the truck will be slowed by the impact. Do I think the decrease in speed would register on the speedometer? No, but it is still a real deceleration.

23. ### hansdaValued Senior Member

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Thanks for this.