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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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Warp wrote:
> Paul Fuller <pgf### [at] optusnet comau> 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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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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Darren New wrote:
> 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. ;-)
>
Is that with or without buttered toast attached to its back :)
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Paul Fuller <pgf### [at] optusnetcomau> wrote:
> Sorry but you are just wrong.
It's easy to say "you are just wrong" without actually explaining why.
> 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.
Attach the cable to the opposite side of the parent body. What happens?
Besides, we can just forget the cable: Simply shoot the projectile and
that's it. With the correct amount of speed it will stop the object from
rotating. Where did the angular momentum go?
> No system of cables, pulleys, magnets, gyroscopes, friction,
> electricity, radiation, carbon nonotubes, superconductors or whatever
> can alter that.
Since a spinning object can be used to produce energy (eg. by friction)
you are effectively saying that a spinning object is an infinite source
of energy because its angular momentum will never disappear.
--
- Warp
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Paul Fuller <pgf### [at] optusnetcomau> wrote:
> > 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.
So you are effectively saying that regardless of heat produced by
friction, angular momentum is always conserved. This would effectively
make a spinning object an infinite source of energy.
> > This would be true in a completely friction-free system. The thing is,
> > friction dissipates part of this energy.
> Still hung up on friction !
A spinning object can be used to produce heat by friction.
> True enough. Friction is the mechanism that takes energy from the
> spinning Earth.
Where does this energy come from?
> At the same time there is a change elsewhere in the
> system that conserves angular momentum overall.
Which means that the energy was produced completely for free?
Isn't that kind of the definition of a perpetual motion machine?
> 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.
Then what is it that is converted to heat by friction?
> 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.
Two objects with no angular momentum at all collide off-center, and
they get stuck to each other. The resulting union of masses will start
spinning because of the collision.
Somewhere else in the universe something else starts spinning in the
opposite direction due to a magical universal conservation of angular
momentum law.
Yes, I understand perfectly now. Thanks for clearing that up.
--
- Warp
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Warp <war### [at] tagpovrayorg> wrote:
> Besides, we can just forget the cable: Simply shoot the projectile and
> that's it. With the correct amount of speed it will stop the object from
> rotating. Where did the angular momentum go?
I thought about this and became to a conclusion. You could have explained
it if you knew it instead of just saying "you are wrong" without any
explanation.
The answer is that the object-projectile system still has the angular
momentum. If we calculate the angular momentum of this system after the
firing, ie. the how the system is oriented with regard to the center of
mass of the system and the distance between the two objects, we will
probably get an angular momentum equivalent to the original one.
The same is probably true for two approaching objects which collide.
Even though each object by itself didn't have any angular momentum, the
two-object system did. The entire two-object system is actually rotating
around the center of mass of the two objects (even though they two objects
are travelling almost rectilinearly; this is because they are not travelling
along the same line in space). When they collide and stick to each other,
the "speed of rotation" they had just before they collided will be kept. The
angular momentum will be unmodified. Only if the two objects were travelling
exactly on the same line in space will there be no rotation because there's
no angular momentum.
--
- Warp
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scott <sco### [at] laptopcom> wrote:
> > http://www.xkcd.com/162/
> "Each turn robs the planet of angular momentum"
> Unfortunately not...
Maybe the idea was that *while* she is spinning, the Earth rotates more
slowly, making the day longer? When she stops spinning the rotation of the
Earth may return to normal, but that day will still have been longer
nevertheless.
--
- Warp
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On Sun, 28 Oct 2007 11:00:57 +0100, Warp <war### [at] tagpovrayorg> wrote:
> Maybe the idea was that *while* she is spinning, the Earth rotates more
> slowly, making the day longer? When she stops spinning the rotation of
> the
> Earth may return to normal, but that day will still have been longer
> nevertheless.
Yeah, but she will have spent it spinning...
--
FE
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Warp wrote:
> Warp <war### [at] tagpovrayorg> wrote:
>> Besides, we can just forget the cable: Simply shoot the projectile and
>> that's it. With the correct amount of speed it will stop the object from
>> rotating. Where did the angular momentum go?
>
> I thought about this and became to a conclusion. You could have explained
> it if you knew it instead of just saying "you are wrong" without any
> explanation.
>
> The answer is that the object-projectile system still has the angular
> momentum. If we calculate the angular momentum of this system after the
> firing, ie. the how the system is oriented with regard to the center of
> mass of the system and the distance between the two objects, we will
> probably get an angular momentum equivalent to the original one.
>
> The same is probably true for two approaching objects which collide.
> Even though each object by itself didn't have any angular momentum, the
> two-object system did. The entire two-object system is actually rotating
> around the center of mass of the two objects (even though they two objects
> are travelling almost rectilinearly; this is because they are not travelling
> along the same line in space). When they collide and stick to each other,
> the "speed of rotation" they had just before they collided will be kept. The
> angular momentum will be unmodified. Only if the two objects were travelling
> exactly on the same line in space will there be no rotation because there's
> no angular momentum.
>
Several times, as carefully as I could I explained that angular momentum
is conserved in a closed system. That is a general principle that is
apparently fundamental to the way the universe works. Apart from
stating the principle which you will find in any physics textbook that
touches on the subject and on many websites, I also explained as
carefully as I could in this medium where the angular momentum was
transferred from and to in several examples.
Sorry but you continued to assert that friction could cancel out angular
momentum and that a spinning closed system could be brought to rest
without any external force.
When I asked plainly is angular momentum conserved in a closed system
you replied:
>>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).
I'm still unclear from your other posts as to whether you still maintain
this position. Perhaps you have taken some time to research the facts
and would recant your statement?
I'll admit to being a poor instructor. I think you have to admit to
being a) wrong and b) pig-headed about it.
I will reply to your other posts to try to clear up some of the wrong
assertions that you continue to make in them. It would be nice if you
could reply in one coherent go though rather than in dribs and drabs of
silliness.
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