Paul Fuller <pgf### [at] optusnetcomau> wrote:
> >   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.

  In order for the rotating secondary object to affect the primary object's
rotation, it has to be connected to the primary object somehow. This
connection causes friction.

> >   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.

  This would be true in a completely friction-free system. The thing is,
friction dissipates part of this energy.

> >   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.

  I have no idea what cartoon you are talking about.

  Anyways, in the Earth-Moon system (and in fact, in any planet-moon
or star-planet, or basically any object-orbits-another-object system)
the slowdown of the rotation of the Earth is caused by tidal forces
caused by the Moon. In practice it means that the Moon deforms the
Earth as the Earth rotates, and this deforming produces heat (because
of friction) which is dissipated. This heat energy is "robbed" from
somewhere: The angular momentum of the Earth.

  It's the reason why the Moon always shows us the same side. It has not
always been like that, but it has become like that because of tidal forces
slowing it down.
  In the Pluto-Kharon system this is even more accentuated, as they both
orbit each other synchronously, each one showing the other always the
same side. It has not always been so.

  So, you see, even in a closed system rotation can be stopped without
ejecting any material, just by converting angular momentum into heat.

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

  Angular momentum can be lost by converting it to heat (or other forms
of energy for that matter), so the answer is no, unless you don't consider
it a "closed system" anymore if there's heat dissipation (OTOH, this heat
could theoretically be collected and stored, keeping the whole thing a
closed system).

> 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.

  How do you think things started rotating in the first place?

  And what is your explanation of why planets and moons are slowing down?

-- 
                                                          - Warp

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