Warp wrote:
> Paul Fuller <pgf### [at] optusnetcomau> wrote:
>> As the part is spun up, the larger part is spun in the opposite 
>> direction to a degree determined by their relative masses.  The net 
>> angular momentum of the system stays constant assuming that this is a 
>> closed system.
> 
>   You assume that the rotation can be done without any friction. This is,
> in fact, impossible in practice.
No I don't.  Friction is not the issue.  Wherever the energy comes from 
- say a battery or solar panel - is irrelevant.  Where it goes to - heat 
via friction or some of it converted back into electricity - doesn't matter.
> 
>   Friction produces heat. Heat is energy. This energy must come from
> somewhere.
Energy was stored in the rotating masses.  You stop them rotating then 
you get the energy back in one form or another.  But you can't just stop 
one of the masses rotating.  There is an opposite effect on the other mass.
> 
>   Even if the Earth-Moon system was a completely isolated closed system
> in space, Earth's rotation would still slow down. Why? (Granted, the
> situation is not identical, but the basic cause for the slowdown is.)
> 
If you are referring to the cartoon then the Earth would be affected 
because a small part (the girl) starts rotating by pushing against it. 
The Earth's rotation is altered ever so slightly in the opposite 
direction.  While she spins at a constant rate the effect on the Earth 
stays the same.  Since she will experience friction she has to add 
energy to keep spinning at the same rate.  However even if she 
experienced lower friction or none at all it does not matter to the 
rotation so long as she adds energy to stay rotating at the same rate.

When she stops spinning either by allowing friction to do its job or by 
actually exerting a force herself then either way a force acts 
ultimately on the Earth in the opposite direction.

There is a simple statement that you either agree with or not: "Angular 
momentum in a closed system is conserved".  Yes or No ?

If 'No' then please provide an explanation or link explaining how any 
form of energy can be turned into angular momentum in a closed system.

A Nobel prize in physics and immense wealth awaits.

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