Bill Pragnell wrote:
> Darren New wrote:
>> (The railguns in the story actually had to fire for a few thousand 
>> rounds to punch a hole in the atmosphere so the needles wouldn't just 
>> vaporize.)
> 
> That's pretty cool. Relativistic weapons would only really be as useful 
> as nukes - to cause almost-total destruction.

I've seen that too - drop stuff from orbit. These were basically 
anti-armor weapons. Attacking buildings and planes and satellites (from 
ground) and tanks and stuff like that. They felt bad when they got 
attacked indoors and had to shoot at the attacker with a bunch of 
unarmored enemy support troops about, because they all got smeared to 
red goo. :-)

Fun books.

> You'd have to design any such gun to be recoilless somehow. 

Generally by firing something out the back at a similar momentum. Kind 
of like the open back on a shoulder-launched rocket/bazooka thing.

-- 
   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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Sabrina Kilian wrote:
>>> Yep. If it can go thru a concrete wall coming out the front, the stock
>>> can certainly break your shoulder.

> I think it would depend on the mass of the projectile, otherwise
> headlights on cars would be pretty useless.

Well, they don't go through concrete, do they? Certainly mine dont!


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

Post a reply to this message

Bill Pragnell wrote:
>>> or the dubious claim that it can fire projectiles at near lightspeed 
>>
>> That's what a railgun is for.
> 
> No it's not. A railgun accelerates a projectile with something like a 
> linear accelerator. You get a fast-moving slug but I don't think 
> anyone's under the illusion that it could be relativistic.

I think it's more that it's the only technology that's likely to be able 
to accelerate something to relativistic speeds in a reasonable "barrel 
length."  Certainly an explosives-based lump-in-a-tube cannon won't.

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

Post a reply to this message

Orchid XP v7 wrote:
> There's 300 enemy troops with SMGs, grenades and rocket launchers, not 
> to mention 3 helicopters, and yet 1 lone commando manages to shoot them 
> all down without dying?

That's what always un-immerses me in FPS games. In Quake II (IIRC) you 
wind up basically killing the entire enemy army yourself without ever 
seeing another living teammate.

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

Post a reply to this message

>> Bolt it to the Earth, then (assuming you're measuring recoil relative to
>> the Earth) the recoil will be avoided.
>
> http://www.xkcd.com/162/

"Each turn robs the planet of angular momentum"

Unfortunately not...

Post a reply to this message

Darren New wrote:
> Bill Pragnell wrote:
>> You'd have to design any such gun to be recoilless somehow. 
> 
> Generally by firing something out the back at a similar momentum. Kind 
> of like the open back on a shoulder-launched rocket/bazooka thing.

That's the only method I've ever heard of, but I can't imagine it would 
work well for a hand-held relativistic projectile weapon! You'd end up 
standing at the cusp between two lobes of utter annihilation.

Post a reply to this message

Darren New wrote:
> Bill Pragnell wrote:
>>>> or the dubious claim that it can fire projectiles at near lightspeed 
>>>
>>> That's what a railgun is for.
>>
>> No it's not. A railgun accelerates a projectile with something like a 
>> linear accelerator. You get a fast-moving slug but I don't think 
>> anyone's under the illusion that it could be relativistic.
> 
> I think it's more that it's the only technology that's likely to be able 
> to accelerate something to relativistic speeds in a reasonable "barrel 
> length."  Certainly an explosives-based lump-in-a-tube cannon won't.

Maybe, but only if it's a very small projectile, and probably only 
useful in a vacuum (see your friction-vapourisation-explosion discussion 
above).

According to a quick shufti on google, a particle with a rest mass of 
1mg would need to be given 4.5 MJ of kinetic energy to get it to 0.01c, 
and about 450 MJ for 0.1c. I guess that's feasible as a weapon...

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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.

-- 
                                                          - Warp

Post a reply to this message

>> "Each turn robs the planet of angular momentum"
>
>> Unfortunately not...
>
>  I don't see, technically speaking, why not.

Because you need angular *acceleration* to "rob the planet of momentum", not 
angular velocity.

Simply spinning at a constant 5000rpm is not going to change the planet's 
momentum, it will stay constant no matter how many times you turn or for how 
long you stay turning.

Of course spinning up to 5000rpm from 0rpm will change the momentum, but 
then spinning back down from 5000 to 0 will have precisely the opposite 
effect.

So, each turn, does not in fact, rob the planet of any momentum.

Post a reply to this message

scott <sco### [at] laptopcom> wrote:
> Because you need angular *acceleration* to "rob the planet of momentum", not 
> angular velocity.

> Simply spinning at a constant 5000rpm is not going to change the planet's 
> momentum, it will stay constant no matter how many times you turn or for how 
> long you stay turning.

  Take into account air friction and friction from the ground. In order to
maintain the constant turning speed you have to actually accelerate
constantly due to these frictions.

  Basically you can't have anything connected to the plane rotating at
a constant velocity without applying a force to it (because of friction).
And an applied force means acceleration.

-- 
                                                          - Warp

Post a reply to this message

>> Because you need angular *acceleration* to "rob the planet of momentum", 
>> not
>> angular velocity.
>
>> Simply spinning at a constant 5000rpm is not going to change the planet's
>> momentum, it will stay constant no matter how many times you turn or for 
>> how
>> long you stay turning.
>
>  Take into account air friction and friction from the ground. In order to
> maintain the constant turning speed you have to actually accelerate
> constantly due to these frictions.
>
>  Basically you can't have anything connected to the plane rotating at
> a constant velocity without applying a force to it (because of friction).
> And an applied force means acceleration.

Friction against what?  Newton's 3rd law...

Post a reply to this message

scott <sco### [at] laptopcom> wrote:
> Friction against what?  Newton's 3rd law...

  Against the ground and the air.

  Friction causes heat, and the energy for that heat must come from somewhere.
I cannot be produced from nothing.

  The answer is that the energy which produces that heat is coming from
the angular momentum, which is thus reduced by a proportional amount.

-- 
                                                          - Warp

Post a reply to this message

Darren New wrote:
> Orchid XP v7 wrote:
>> There's 300 enemy troops with SMGs, grenades and rocket launchers, not 
>> to mention 3 helicopters, and yet 1 lone commando manages to shoot 
>> them all down without dying?
> 
> That's what always un-immerses me in FPS games. In Quake II (IIRC) you 
> wind up basically killing the entire enemy army yourself without ever 
> seeing another living teammate.

An entire army is sent to the alien homeworld. The aliens wipe out all 
but 1 soldier, who is left armed with only a tiny pistol. Yet he alone 
is able to destroy an entire string of tactically significant targets, 
completely unaided.

Hmm. :-)

I guess the idea is to massage your ego by letting you be the 
super-soldier that nobody can kill.

Neat intro tho...

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Warp wrote:

> I cannot be produced from nothing.

That's deep...

Post a reply to this message

Orchid XP v7 escribió:
> Warp wrote:
> 
>> I cannot be produced from nothing.
> 
> That's deep...

:D

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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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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

Post a reply to this message

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.

Post a reply to this message

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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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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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?

Post a reply to this message

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.

Post a reply to this message

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!

Post a reply to this message

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 :)

Post a reply to this message

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

Post a reply to this message

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

Post a reply to this message

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

Post a reply to this message

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

Post a reply to this message

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

Post a reply to this message

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.

Post a reply to this message

Warp wrote:
> 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.
I actually do explain more after that one line that you quoted.
> 
>> 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?
When the cable goes taut the projectile will exert a force on the body 
(and vice versa) that will be off centre and will cancel the rotation. 
Of course things will bump around but once that is finished the net 
rotation will be as before the projectile was fired.  At every stage if 
you care to add up the angular momentum of the system it will be constant.

I can't reply with numbers and formulae unless you can give a diagram 
with distances, masses and velocities.
> 
>   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?
It is carried in the off centre velocity of the mass of the projectile.
> 
>> 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.
> 
Absolutely not and it is revealing that you misunderstand or 
misrepresent the argument to that degree.  I'm not talking about the 
rotation of any single part of the system being constant.  I'm talking 
about the sum of the angular momentum of the whole system.

As the rotation of a mass A is reduced, an opposite change in angular 
momentum occurs somewhere else in the system.  And not by magic.  simply 
ask what is exerting the tangential force to slow down A and you'll find 
an equal and opposite force acting on that other thing.  And please 
don't say friction is exerting the force!  Some other part of the system 
must be applying the force be it via friction or any other means.

That you make just nonsense statements on my behalf then tear them down 
is poor style.

Post a reply to this message

Warp wrote:
> 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.
I am saying that angular momentum is conserved.  As I have replied 
elsewhere to you this does not make your tacked on sentence about 
infinite energy true.  I am not saying that but you have added it 
because you misunderstand or want to misrepresent what I am saying.
> 
>>>   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.
Yes.  So can rubbing sticks together.  Neither will in any case alter 
the total angular momentum of a system.
> 
>> True enough.  Friction is the mechanism that takes energy from the 
>> spinning Earth.
> 
>   Where does this energy come from?
There is a spinning mass.  That stores energy in kinetic form.  Slowing 
it down by friction converts stored kinetic energy to heat energy.  It 
does not convert angular momentum into heat.  There will be an effect 
elsewhere that causes a matching change to the angular momentum of 
another part of the system.  Look I even said that in the next paragraph.
|
V
> 
>>  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?
No it is transferred.  Please stop making silly assertions on my behalf.
> 
>> 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?
Energy stored in the rotation of the Earth, Moon and both around each 
other.  Lots of energy there!  And if you care to read on the subject 
you'll find that the rotation of the Earth has slowed and the separation 
of the Earth and Moon has increased over time.  The energy has been 
transferred into heat by friction as you say.
> 
>> 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.
As I think you have realised elsewhere the system comprising the 
original two masses does already have angular momentum.  This then shows 
up as the rotation of the combined body.
> 
>   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.
> 
No.  You are just being silly again trying to paint my arguments and 
make them appear ridiculous.  There is no magic and something 
unconnected does not start spinning.  Any change to the rotation of one 
part of a system must be caused by a force exerted on it by another 
part.  That is where you will find the corresponding change in rotation 
that balances out the angular momentum.

Post a reply to this message

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.
So now you're saying that *I* don't understand the topic.  Very, very rich.
> 
>   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.
Sounds like you are coming around to the correct view that total angular 
momentum is conserved.
> 
>   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.
> 
Yes.  Angular momentum is conserved.

Post a reply to this message

Paul Fuller wrote:
> 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. 

Well, it's not just random. All the "conservation" laws are based on 
"symmetry" laws.  Conservation of energy is based on the premise that 
what you do here will act the same as what you do there. Conservation of 
momentum follows from the idea that an experiment you do now will give 
the same results as an experiment you do later. Conservation of angular 
momentum is based on the premise that an experiment you do facing this 
way will give the same results as an experiment facing that way.

There are similar relationships in the quantum world like conservation 
of spin and so on.

I.e., it's not just that it happens to hold. We know *why* it happens to 
hold, and how to tell when we find a situation where it doesn't hold.

Which I personally find pretty cool to know. :-)

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

Post a reply to this message

Fredrik Eriksson wrote:
> Yeah, but she will have spent it spinning...

"Only God can make a tree? And what of acorns, then? Are we throwing out 
biology?"  ;-)

You're all nerds.

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

Post a reply to this message

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

> Several times, as carefully as I could I explained that angular momentum 
> is conserved in a closed system.

  No, you didn't explain anything. You just stated something. More
specifically, I don't remember you explaining precisely how angular
momentum is preserved in the specific case I quoted above (which is
what I asked about).

  Basically your "explanation" was "the angular momentum is conserved
because angular momentum is always conserved". That's not an explanation
of how angular momentum is conserved in the example above.

> 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.

  You have still failed to explain why friction does not reduce angular
momentum. You have simply stated that it doesn't.

  What I don't understand is how a rotating object can produce heat but
still maintain its full angular momentum. You have not explained this
at all.

> I'll admit to being a poor instructor.  I think you have to admit to 
> being a) wrong and b) pig-headed about it.

  That kind of language is indeed not going to receive too much
approval from me.

  Basically what you have stated is "angular momentum is always preserved
because that's how it works, you are wrong, you are stubborn, you are
silly". That is supposed to convince me about anything?

> 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.

  That kind of attitude and language is not the best possible to get a
positive attitude from my part.

  I want rational explanations, not "you are wrong because you are wrong"
type of null statements.

-- 
                                                          - Warp

Post a reply to this message

Paul Fuller <pgf### [at] optusnetcomau> wrote:
> 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.
> So now you're saying that *I* don't understand the topic.  Very, very rich.

  OMG. You are deliberately making this into a flamewar.

  Ok, I'll stop, because I'm too prone to get flamed.

  You can think whatever you want.

-- 
                                                          - Warp

Post a reply to this message

Warp wrote:
>   What I don't understand is how a rotating object can produce heat but
> still maintain its full angular momentum. You have not explained this
> at all.

Angular momentum consists of both velocity and distance. When the skater 
pulls her arms in and speeds up, the muscular energy turns into kinetic 
energy, but the angular momentum stays the same. When the spinning disk 
rubs against the disk spinning the other way, the kinetic energy of the 
disks is turned into the kinetic energy of the individual atoms (i.e., 
heat), but the positive-signed spinning of the top disk cancels the 
negative-signed spinning of the bottom disk.

So, you can change energy without changing angular momentum by spinning 
faster but closer, or by having two things spinning opposite directions 
change their rate of spin the same amount in different directions.

Energy, on the other hand, isn't signed (except in some rather bizarre 
circumstances, and to make the math come out for potential energy in 
situations that never actually occur in reality), so that's how it can 
happen that you can change the form of energy without changing the 
amount of angular momentum.

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

Post a reply to this message

Darren New wrote:
> Paul Fuller wrote:
>> 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. 
> 
> Well, it's not just random. All the "conservation" laws are based on 
> "symmetry" laws.  Conservation of energy is based on the premise that 
> what you do here will act the same as what you do there. Conservation of 
> momentum follows from the idea that an experiment you do now will give 
> the same results as an experiment you do later. Conservation of angular 
> momentum is based on the premise that an experiment you do facing this 
> way will give the same results as an experiment facing that way.
> 
> There are similar relationships in the quantum world like conservation 
> of spin and so on.
> 
> I.e., it's not just that it happens to hold. We know *why* it happens to 
> hold, and how to tell when we find a situation where it doesn't hold.
> 
> Which I personally find pretty cool to know. :-)
> 
I don't think I implied that it is random.  If so then let me clear that up.

What I said was 'apparently fundamental to the way the universe works'. 
    Just my choice over declaring it a 'law'.  Certainly not random or 
by accident.

We can understand that "you can't create something out of nothing" or 
"everything must be balanced".  Statements of principles that help us to 
investigate and understand everything else.

I don't think you can explain though why those principles are true 
without essentially coming back to restating them or observing that 
we've never seen them to be broken.  So I'll call them fundamental. 
They are the rules of the game.  The pieces follow the rules not because 
they want to but because there simply is no way not to.

Sure they match our sense of fair play.  Perhaps even our sense of fair 
play is based on the fundamental principle rather than the other way 
around ?

When you say the "Conservation of energy is based on the premise ..." 
and "Conservation of momentum follows from the idea ...", I would 
instead say that the premise and the idea are based on the fundamental 
principles.

And it is cool :)

Post a reply to this message

Darren New <dne### [at] sanrrcom> wrote:
> Angular momentum consists of both velocity and distance. When the skater 
> pulls her arms in and speeds up, the muscular energy turns into kinetic 
> energy, but the angular momentum stays the same. When the spinning disk 
> rubs against the disk spinning the other way, the kinetic energy of the 
> disks is turned into the kinetic energy of the individual atoms (i.e., 
> heat), but the positive-signed spinning of the top disk cancels the 
> negative-signed spinning of the bottom disk.

  If we express that in overly simple terms: If a rotating system consists
of several parts, bringing those parts closer together requires energy.
If those parts are later pulled apart, that energy is released?

  Or perhaps in another way: Bringing more variation to local spinning
at different parts of the system requires energy, but evening out the
local variations and bringing the whole system to a more even state
(with less local variations in spin) releases that energy?
  (In other words, in a closed system getting two discs to rotate
independently in the same direction requires energy. Colliding those
discs so that they will start rotating as one single object will release
that energy?)

-- 
                                                          - Warp

Post a reply to this message

You've selectively quoted the bits where I have been peeved with your 
lack of comprehension after several attempts.  All the bits where I have 
tried to explain to you the underlying physics seem to have passed right 
by you.

So I'll forget everything else and just try to explain the heart of it.

You misunderstand what angular momentum is.  Stop thinking that the 
angular momentum of a system is just the one part that is obviously 
spinning around.

The angular momentum of a system is the sum of the mass * velocity * 
radius of every single part of the system all taken relative to some 
reference point.

Mass and velocity should be clear.

Radius is the perpendicular distance from the reference point to the 
velocity vector.  Thus if the velocity vector passes exactly through the 
reference point the contribution to angular momentum is zero.  Any 
particle in the system that is moving and the velocity vector does not 
point exactly through the reference point contributes angular momentum 
to the total.  Now the contribution can be positive or negative 
depending on the sign of the velocity and which side of the reference 
point it falls on.

You can choose any reference point so long as it remains fixed.  It is 
often convenient to take the centre of mass of the system as the 
reference point when considering angular momentum.  Note that the centre 
of mass of a closed system is fixed within the frame of reference of 
that system.

Consider a closed system consisting of mass A that is not spinning and 
object B that is spinning.  Suppose A is the fixed frame of a satellite 
and B is the spinning wheel of a gyroscope.  All of the angular momentum 
of the system is contributed by the spinning mass B for now.  Let the 
total angular momentum be K.  K is currently not zero and lets say we 
choose our units and reference point etc so that it comes out as +100 
units.  The SI unit for angular momentum is Newton metre seconds (Nms) 
or Kgm^2s^-1.

Note that B possesses both angular momentum and kinetic energy.

Now apply a brake between A and B that brings them completely to rest 
with respect to each other.  The kinetic energy is converted heat.  As 
far as this closed system is considered there is no more usable kinetic 
energy available it has all been converted to heat.

Where is the angular momentum?  Well in applying a braking force to B, 
object A has experienced an equal and opposite force.  This causes it to 
start spinning.  In fact the whole system is now spinning compared to an 
external reference system.

The spin is in the same direction as the object B was spinning.  But A+B 
is not spinning at the same rate as B was alone.

How fast is it spinning and what is the new total angular momentum? 
Well it is spinning exactly fast enough so that calculating the mass * 
velocity * radius of every particle and adding them up comes to K as 
before.  Exactly.  There is no conversion of angular momentum to or from 
any form of energy.

Saying that K remains the same is not saying that there is any usable 
kinetic energy left in the system.  So forget the idea of perpetual motion.

The system had net angular momentum of K at the start, the end and at 
every intermediate point.

Considering it the opposite way around, if a satellite is spinning it 
can start to spin some part of itself in the opposite direction.  If 
this is done precisely enough then the satellite stops spinning but only 
so long as it keeps the gyroscope spinning at that speed and direction. 
  As friction slows down the gyroscope the satellite will start to spin 
again.  It can keep pumping in energy to maintain the rate of spin to 
keep itself pointed in one direction.  So energy keeps having to be 
introduced and it comes back out as heat due to friction.  This does not 
at any time alter the total angular momentum of the system.  AM has been 
transferred to the gyroscope and is stored there to come back out later.

If on the other hand thrusters are used to halt the rotation then the 
expelled particles carry away the angular momentum.  They are masses 
moving with velocity at a perpendicular distance to the reference point. 
    The particles in the thruster stream themselves don't need to be 
spinning to carry angular momentum relative to the reference point by 
the way.

Angular momentum is something different to energy and has its own 
accounting ledger.  The ledger always has to balance and you can't 
transfer amounts from the angular momentum ledger to or from the energy 
ledger.  Same with linear momentum.  It has its own ledger that always 
balances and likewise you can't transfer into or out of it from either 
energy or angular momentum.

Post a reply to this message

Paul Fuller <pgf### [at] optusnetcomau> wrote:
> If on the other hand thrusters are used to halt the rotation then the 
> expelled particles carry away the angular momentum.

  A theoretical question: Can mass be converted to other forms of energy?
(I think GR said something about this?)

  Assuming yes, how does this conversion affect angular momentum?

-- 
                                                          - Warp

Post a reply to this message

Warp wrote:
> Paul Fuller <pgf### [at] optusnetcomau> wrote:
>> If on the other hand thrusters are used to halt the rotation then the 
>> expelled particles carry away the angular momentum.
> 
>   A theoretical question: Can mass be converted to other forms of energy?
> (I think GR said something about this?)
Yes.
> 
>   Assuming yes, how does this conversion affect angular momentum?
> 
Simple - a photon carries angular momentum.

Post a reply to this message

Paul Fuller wrote:
> I don't think I implied that it is random.  If so then let me clear that 
> up.

Sorry. Random is a bad word to use.

That things work the same no matter which way you face is the "random" 
part. It's not necessary that things work that way. But it seems they 
do. *Given* that, conservation of angular momentum is a result.

> I don't think you can explain though why those principles are true 
> without essentially coming back to restating them or observing that 
> we've never seen them to be broken.  

Right. What I was trying to express is that "angular momentum is 
conserved" isn't really the "we've never seen it to be broken" part. 
It's the "experiments run the same no matter which way you face" that's 
the "never seen to be broken part".  Conservation of angular momentum is 
an effect, not a cause.

> And it is cool :)

Yup.

-- 
   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:
>   If we express that in overly simple terms: If a rotating system consists
> of several parts, bringing those parts closer together requires energy.

Yes.

> If those parts are later pulled apart, that energy is released?

Assuming it hasn't dissipated somewhere, yes.  Think of spinning around 
an axle, reeling in a weight on a string. Now let the weight pay out, 
and you can make it do work like play a music box.

>   Or perhaps in another way: Bringing more variation to local spinning
> at different parts of the system requires energy,

I don't think the spinning and the energy are particularly connected. If 
you're talking about closed systems, the energy doesn't go away, it just 
moves around. It may move to a place (like heat) from which you can't 
move it back.

>   (In other words, in a closed system getting two discs to rotate
> independently in the same direction requires energy. Colliding those
> discs so that they will start rotating as one single object will release
> that energy?)

I don't think that's right, no. In a closed system, you can move energy 
around but not create or destroy it. Spinning them in opposite 
directions requires moving energy from somewhere else, like a spring or 
a chemical explosion or something. In a closed system, you can't get 
them both rotating in the same direction without something else rotating 
in a different direction.

-- 
   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:
>   A theoretical question: Can mass be converted to other forms of energy?

Mass *is* energy. That's where energy is stored. That's why things with 
lots of kinetic energy are heavier.  That's why a helium atom weighs 
less than four hydrogen atoms.   E=mc^2.

>   Assuming yes, how does this conversion affect angular momentum?

It doesn't. It doesn't go anywhere.  It's all the same stuff.

-- 
   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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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> >   A theoretical question: Can mass be converted to other forms of energy?

> Mass *is* energy. That's where energy is stored. That's why things with 
> lots of kinetic energy are heavier.  That's why a helium atom weighs 
> less than four hydrogen atoms.   E=mc^2.

  That didn't really answer my question.

> >   Assuming yes, how does this conversion affect angular momentum?

> It doesn't. It doesn't go anywhere.  It's all the same stuff.

  Since angular momentum is dependent on mass, that means that eg. heat
has mass?

-- 
                                                          - Warp

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Paul Fuller <pgf### [at] optusnetcomau> wrote:
> >   Assuming yes, how does this conversion affect angular momentum?
> > 
> Simple - a photon carries angular momentum.

  Is electromagnetic radiation the only possible form of energy?

-- 
                                                          - Warp

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Warp wrote:
> Paul Fuller <pgf### [at] optusnetcomau> wrote:
>>>   Assuming yes, how does this conversion affect angular momentum?
>>>
>> Simple - a photon carries angular momentum.
> 
>   Is electromagnetic radiation the only possible form of energy?
> 
No.

Are you being obtuse ?

Let me re-phrase that.  You are being obtuse.  Is it deliberate ?

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Paul Fuller <pgf### [at] optusnetcomau> wrote:
> Warp wrote:
> > Paul Fuller <pgf### [at] optusnetcomau> wrote:
> >>>   Assuming yes, how does this conversion affect angular momentum?
> >>>
> >> Simple - a photon carries angular momentum.
> > 
> >   Is electromagnetic radiation the only possible form of energy?
> > 
> No.

> Are you being obtuse ?

> Let me re-phrase that.  You are being obtuse.  Is it deliberate ?

  Why the hell are you constantly trying to turn this into a flamewar?

  I asked if mass can be converted to other forms of energy, and if this
is so, what happens to the angular momentum. Your answer to this was that
"a photon carries angular momentum". That seems to clearly imply that all
forms of energy consist of photons. When I ask you if what you are trying
to say is that all forms of energy consist of photons you call me obtuse.

  Well, you know what? Go fuck yourself. I'm tired of your condescending
attitude.

-- 
                                                          - Warp

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