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From: Orchid XP v7
Subject: Re: I miss this
Date: 26 Oct 2007 14:06:22
Message: <47222c9e$1@news.povray.org>
Warp wrote:

> I cannot be produced from nothing.

That's deep...


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From: Nicolas Alvarez
Subject: Re: I miss this
Date: 26 Oct 2007 14:57:59
Message: <472238b7@news.povray.org>

> Warp wrote:
> 
>> I cannot be produced from nothing.
> 
> That's deep...

:D


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From: Paul Fuller
Subject: Re: I miss this
Date: 27 Oct 2007 08:30:01
Message: <47232f49@news.povray.org>
Warp wrote:
> scott <sco### [at] laptopcom> wrote:
>> "Each turn robs the planet of angular momentum"
> 
>> Unfortunately not...
> 
>   I don't see, technically speaking, why not.
> 
>   Let's assume we have a big object in a weightless space in vacuum
> rotating. Can this rotation be stopped by the object itself without
> applying external forces?
> 
>   The answer is yes. The most obvious way to do this is to fire up some
> rockets in specific directions. This causes the rotation to slow down,
> basically by expelling material at high speed from the object.
> 
>   A slightly less obvious way is to rotate some significant part of the
> object at a different speed. Why does this slow down the overall rotation?
> It's because part of the angular momentum is converted into heat due to
> friction, and this heat dissipation is taken away from this angular momentum.
> 
Nope.

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.

When the spin is removed as you suggest by friction, slowing the 
'rotating part' applies an equal but opposite force to the main body. 
This results in the assembly as a whole returning to the original rate 
and direction of spin (or rest).  It can however be left pointing in a 
different direction.

Angular momentum is conserved.

The heat dissipated is simply the energy used to spin up the system 
being returned.

What is useful is that the system - say a satellite - can be pointed to 
and kept at different directions and that unwanted rotation can be 
countered by precisely controlling a number of gyroscopes.

Real satellites are subject to external forces like drag, outgassing and 
uneven light pressure that can give them unwanted spin.  Gyroscopes can 
be used to soak this up.  Eventually if the external forces keep 
building up in the same direction the gyroscopes can't be spun up safely 
any more to counter the rotation.  The satellite operator uses thrusters 
to really counter the rotation with off-centre thrust.  At the same time 
the gyroscopes are spun down and returned to a rate of rotation that is 
within a comfort zone and correct to match any residual spin after the 
thruster stops.

When the gyroscopes seize or when the thruster propellant is exhausted 
the satellite could no longer be controlled.  Usually some time before 
this the satellite is retired to a safer parking orbit or in some cases 
is even de-orbited.  The idea is to do this while the operator can 
control the orientation.

Hubble is a good example.  It has 6 large gyroscopes.  Originally 3 were 
used to be able to point and hold very steady in any given orientation. 
   The others were provided as spares.  Quite a few gyroscopes have 
failed and had to be replaced during servicing missions.  IIRC it was 
out of action for a while when 4 gyros failed but they developed a way 
to operate with only 2 but with reduced capability.  At the moment it is 
operating with close to no redundancy and NASA has concluded that a 
shuttle won't be used to perform another service mission.  The proposal 
was to de-orbit it while they have control.  This caused furore and NASA 
was reconsidering.  Last I heard a 5th servicing mission was back on for 
next year.  Pessimistically I'd guess that no mission will happen and 
that good old Hubble is either going to be brought down or otherwise 
become unusable when the next gyro fails.

Another aside - There is some research going on to develop 
micro-thrusters that could potentially do away with gyroscopes in future 
satellites.  Each one provides a once-off and very small amount of 
thrust.  A very large number are built into a grid.  Imagine something 
like a chip built using semiconductor fabrication holding 1000x1000 
little electronically fired cells.  Each contains a microgram of solid 
propellant.  Place lots of these chips at strategic points on the 
outside of the satellite.


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From: Warp
Subject: Re: I miss this
Date: 27 Oct 2007 10:09:22
Message: <47234691@news.povray.org>
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.

  Friction produces heat. Heat is energy. This energy must come from
somewhere.

  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.)

-- 
                                                          - Warp


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From: Paul Fuller
Subject: Re: I miss this
Date: 27 Oct 2007 13:02:28
Message: <47236f24$1@news.povray.org>
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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From: Warp
Subject: Re: I miss this
Date: 27 Oct 2007 13:27:26
Message: <472374fe@news.povray.org>
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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From: Warp
Subject: Re: I miss this
Date: 27 Oct 2007 13:48:27
Message: <472379eb@news.povray.org>
Ok, here's an easy way of stopping the rotation of an object in a closed
system:

  Construct a cannon on the surface of the object so that it will shoot
in the direction of the tangent, towards the rotation direction. Shoot
a heavy-enough object fast enough so that the recoil of this shot will
stop the rotation of the main object. (If you are worried about possible
effects of having just one such cannon on the surface of the object then
just put another identical cannon at the opposite side of the object,
aimed towards the opposite direction, ie. still at the direction of
rotation at that side, and fire both cannons simulatenously.)

  The projectile is attached to a cable connected to the object at a
point where the pull caused by the object when it reaches the cable
length causes an even force on the center of mass of the object. (Such
a point must exist because there are two extremes: Attaching the cable
on the object at the point where the cannon is would effectively negate
the whole operation. However, attaching the cable on the opposite side
of the object would actually make it start rotating in the opposite
direction compared to the original one. A balance point must thus exist
somewhere in between.)

  When the projectile has thus been stopped, just pull it back to the
surface of the object. The rotation of the whole system will have stopped
without permanently ejecting any material.

  (Where did all the angular momentum go? Well, stopping the projectile(s)
will require energy. The majority of this energy will be dissipated as
heat from the cable.)

-- 
                                                          - Warp


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From: Darren New
Subject: Re: I miss this
Date: 27 Oct 2007 16:32:46
Message: <4723a06e@news.povray.org>
Paul Fuller wrote:
> 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.

Hold your cat upside down, then drop him. ;-)

-- 
   Darren New / San Diego, CA, USA (PST)
     Remember the good old days, when we
     used to complain about cryptography
     being export-restricted?


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From: Paul Fuller
Subject: Re: I miss this
Date: 27 Oct 2007 19:56:28
Message: <4723d02c$1@news.povray.org>
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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From: Paul Fuller
Subject: Re: I miss this
Date: 27 Oct 2007 20:12:42
Message: <4723d3fa$1@news.povray.org>
Warp wrote:
>   Ok, here's an easy way of stopping the rotation of an object in a closed
> system:
> 
>   Construct a cannon on the surface of the object so that it will shoot
> in the direction of the tangent, towards the rotation direction. Shoot
> a heavy-enough object fast enough so that the recoil of this shot will
> stop the rotation of the main object. (If you are worried about possible
> effects of having just one such cannon on the surface of the object then
> just put another identical cannon at the opposite side of the object,
> aimed towards the opposite direction, ie. still at the direction of
> rotation at that side, and fire both cannons simulatenously.)
> 
>   The projectile is attached to a cable connected to the object at a
> point where the pull caused by the object when it reaches the cable
> length causes an even force on the center of mass of the object. (Such
> a point must exist because there are two extremes: Attaching the cable
> on the object at the point where the cannon is would effectively negate
> the whole operation. However, attaching the cable on the opposite side
> of the object would actually make it start rotating in the opposite
> direction compared to the original one. A balance point must thus exist
> somewhere in between.)
> 
>   When the projectile has thus been stopped, just pull it back to the
> surface of the object. The rotation of the whole system will have stopped
> without permanently ejecting any material.
> 
>   (Where did all the angular momentum go? Well, stopping the projectile(s)
> will require energy. The majority of this energy will be dissipated as
> heat from the cable.)
> 
Sorry but you are just wrong.  Reminds me of people who have in their 
mind a design for a perpetual motion machine.  They concoct some system 
of levers and weights or magnets that they think will work.  However if 
you do a complete accounting of all of the forces and effects it simply 
does not work.

In this case you assert that there is a point at which the cables can be 
attached that will prevent the stopping action from imparting any spin 
to the parent body.

Would you like to draw a diagram that shows where this point is ?

The projectile will only affect the rotation of the body if it is fired 
at some off-centre tangent.  You will find that any action that brings 
it back to rest with respect to the parent body will cause an equal but 
opposite force also at an off-centre tangent.  This will always negate 
the spin.

No system of cables, pulleys, magnets, gyroscopes, friction, 
electricity, radiation, carbon nonotubes, superconductors or whatever 
can alter that.

As I said if you can some up with some way to convert between energy and 
net angular momentum then you've got a big future ahead!


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