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