Warp wrote:
> 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.
It has to exert a force and I agree that some energy will be lost in any 
practical system.  You are just not getting the point that friction 
itself has nothing to do with angular momentum.

> 
>>>   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.
Still hung up on friction !
> 
>>>   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.
There was a cartoon linked to in a parent post by 'Orchid XP v7' about a 
girl spinning round to slow down the Earth.
> 
>   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.
True enough.  Friction is the mechanism that takes energy from the 
spinning Earth.  At the same time there is a change elsewhere in the 
system that conserves angular momentum overall.  In this case it is that 
the Moon moves slightly further away from the Earth.  Taking into 
account its rotation around the Earth the total angular momentum stays 
the same.  This has been measured and the calculations work out.
> 
>   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.
The tidal locking effect is well known and you have described it 
reasonably well.  However you are wrong to say that angular momentum is 
converted to heat by friction.
> 
>> 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).
It would have been a good idea for you to read up on this before making 
such a clearly false statement.  Read a physics textbook or Google 
'angular momentum'.  Maybe start with something like Wikipedia.

Go ahead and look it up now.

Then come back and correct your statement please.

Angular momentum is a different thing to energy.  Sorry but there is no 
known way to convert one to another.  You can certainly use energy to 
start one mass spinning.  Thing is that there must be an opposite amount 
of angular momentum showing up somewhere else.
> 
>> 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?
By using energy to exert a force.  Equal and opposite force acts on 
something else.  As I keep saying the angular momentum is conserved by 
an effect somewhere else in the closed system.
> 
>   And what is your explanation of why planets and moons are slowing down?
> 
We agree that planets and moons interect and that this can slow down the 
rotation of both bodies.  Guess what - If the Earth was spinning slower 
than the rate that the moon orbits then the effect would be to 'spin up' 
the Earth rather than slow it down.

You're confusing energy stored in masses rotating with angular momentum 
itself.

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