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PostPosted: Wed Feb 08, 2012 21:20 
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dcbwhaley wrote:
Steve wrote:
I know the answer and the associated subtleties, but before I give it I would really like to know DCB's understanding of this.


The momentum of the fly + train system is conserved (since no external force is applied)

I totally agree with all of your answer.
However, I am still perplexed as to the circumstances where momentum isn't conserved whilst energy is, as per your claim.
Now that you have returned, are you sure you don't want to explain?

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PostPosted: Wed Feb 08, 2012 23:50 
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Steve wrote:
However, I am still perplexed as to the circumstances where momentum isn't conserved whilst energy is, as per your claim.
Now that you have returned, are you sure you don't want to explain?


I am quite happy to be proved wrong but consider this gedanken experiment. A sledge with frictionless runners, in a vacuum, starting from the top of a hill and accelerating under gravity down that hill. At the top its momentum is zero. At the bottom its momentum is non-zero. But, in the absence of friction or air resistance, energy is conserved, being converted from gravitational potential to kinetic.

Also, and this is a genuine thirst for knowledge, how does conservation of momentum apply to a charged particle accelerating in an electric field?

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PostPosted: Thu Feb 09, 2012 00:09 
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A sincere thank you for continuing this is the respectful manner.

dcbwhaley wrote:
I am quite happy to be proved wrong but consider this gedanken experiment. A sledge with frictionless runners, in a vacuum, starting from the top of a hill and accelerating under gravity down that hill. At the top its momentum is zero. At the bottom its momentum is non-zero. But, in the absence of friction or air resistance, energy is conserved, being converted from gravitational potential to kinetic.

The forces in this example are resolved as follows:

Say the runners push the sledge to the viewed left, with the sledge falling downwards.

There is a downwards and left sideways force acting on the sledge (gravity and the effect of the leftwards acceleration respectively). The planet still has the same and opposing forces acting on it: thus the planet is accelerated upwards towards the sledge (by a ratio proportional to the masses) due to the same gravitational pull between them, and the surface accelerated rightwards to the opposing side due to it providing the leftwards force to the sledge (again by a ratio proportional to the masses).
Everything still resolves and is conserved.

Assuming the sledge eventually crashes to a stop: the impact accelerates it upwards and the planet downwards (there might be heat and deformation). The vertical momentum is conserved.
The friction of the sledge moving leftwards across the planet surface (gravity and the coefficient of friction) pulls the planet surface leftwards (cancelling the momentum of before) and also accelerates the sledge rightwards, eventually bringing it to a stop. If no friction applies there too, then the planet surface will continue going rightwards and the sledge leftwards with their speeds unchanged (no decelerating force).
Horizontal momentum is conserved in both cases.

dcbwhaley wrote:
Also, and this is a genuine thirst for knowledge, how does conservation of momentum apply to a charged particle accelerating in an electric field?

That's easy! In this case, the clue is that it is accelerating, so there must be forces involved.
The accelerating charged particle has its own e-field, which in turn interacts with the source field source (the other charged particles [or relative lack thereof] - much like two mass points being accelerated together with the force of gravity between them), thus placing the same and opposite force on them, which in turn places a force on whatever has them bound (preventing them from accelerating towards the charged particle), thus accelerating whatever has bound the charged particles (the relative accelerations being proportional to the ratios of the masses).
All is conserved!

How do you think ion thruster (wiki) engines work? Do those rockets stay still when ejecting the ions? That would be a bit pointless ;)


I've completely answered all of your questions. Can you provide your description of the circumstances where momentum isn't conserved whilst energy is, as per your claim.

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PostPosted: Thu Feb 09, 2012 12:09 
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Steve wrote:
A sincere thank you for continuing this is the respectful manner.

My pleasure :D

Our difference seem to be merely that we disagree on what a "system". I willing accept that if your system is the entire universe then momentum is conserved - though you have to modify the definition of momentum to allow for relativistic effects.

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That's easy! In this case, the clue is that it is accelerating, so there must be forces involved.
The accelerating charged particle has its own e-field, which in turn interacts with the source field source (the other charged particles [or relative lack thereof] - much like two mass points being accelerated together with the force of gravity between them), thus placing the same and opposite force on them, which in turn places a force on whatever has them bound (preventing them from accelerating towards the charged particle), thus accelerating whatever has bound the charged particles (the relative accelerations being proportional to the ratios of the masses).
All is conserved!


Almost!! You are, and in good company with Newton, assuming that all interactions occur instantaneously That is true of gravity (which fundamentally isn't a force) but not of electrical forces. If an electric charge at one location is moved the effect on another charge a foot away will not occur until one nano second later. In that one nano second the momentum of the two particles don't add up. But, I am sure you know the answer to that and that it leads to some very interesting physics :-)

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PostPosted: Fri Feb 10, 2012 00:11 
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dcbwhaley wrote:
Our difference seem to be merely that we disagree on what a "system". I willing accept that if your system is the entire universe then momentum is conserved - though you have to modify the definition of momentum to allow for relativistic effects.

That's not our difference; I will return to this below.
Besides, for me, a system is any system that can be closed; the "entire universe" is merely a SUPERSET of this.

dcbwhaley wrote:
Almost!! You are, and in good company with Newton, assuming that all interactions occur instantaneously That is true of gravity (which fundamentally isn't a force) but not of electrical forces. If an electric charge at one location is moved the effect on another charge a foot away will not occur until one nano second later. In that one nano second the momentum of the two particles don't add up. But, I am sure you know the answer to that and that it leads to some very interesting physics :-)

You almost raise a very valid point, but then I know "forces" are only simplified mathematical models (kinda like imaginary numbers) of some very interesting physics indeed.
If I remember correctly, one of the latest theories is that fields are actually emissions of physical things... So it is wrong to say momentum isn't conserved!

Light has no mass, yet it still holds and can transfer energy. That's another interesting case that seems to defy the classical physics of energy. Indeed, the light energy is supposedly a combination of E and H fields, which are the same "forces" that you used to cast doubt on the conservation of momentum, thus energy can also has the relativistic effects you mentioned (which is obvious) and the same pitfalls that you were alluding to.

So again, I pose to you that you cannot make a distinction between 'conservation of momentum' and 'conservation of energy' 8-)

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PostPosted: Fri Feb 10, 2012 00:51 
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Steve wrote:
Besides, for me, a system is any system that can be closed; the "entire universe" is merely a SUPERSET of this.
What exactly do you mean by closed? I take that to mean that no forces from outside the system act on the bodies within the system. If such forces do exist then the rate of change of momentum of the system is equal to the sum of the applied forces.

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You almost raise a very valid point, but then I know "forces" are only simplified mathematical models (kinda like imaginary numbers) of some very interesting physics indeed. If I remember correctly, one of the latest theories is that fields are actually emissions of physical things... So it is wrong to say momentum isn't conserved!


No really a recent theory. The standard answer to my question is to attribute momentum to the field, just the right amount to conserve the total momentum. Sounds like a fudge but to works.

Quote:
Light has no mass, yet it still holds and can transfer energy. That's another interesting case that seems to defy the classical physics of energy.


Yes. Once you accept the idea of field momentum then light, being a field, has momentum and energy. But you are straying into quantum mechanics where momentum is defined quite differently.

Quote:
So again, I pose to you that you cannot make a distinction between 'conservation of momentum' and 'conservation of energy' 8-)

Insomuch as classical momentum conservation is a purely mechanical concept whereas energy takes other non-mechanical forms I think that there is :D

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PostPosted: Fri Feb 10, 2012 02:07 
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dcbwhaley wrote:
What exactly do you mean by closed?

I have already indirectly answered this: "if an external force is applied to a system, then the system isn’t isolated; otherwise the momentum is conserved.", which for all intents and purposes reflects your description.

dcbwhaley wrote:
The standard answer to my question is to attribute momentum to the field, just the right amount to conserve the total momentum. Sounds like a fudge but to works.

Which is exactly what I did, except I bypassed the concept of the "field". Conserved!

dcbwhaley wrote:
Insomuch as classical momentum conservation is a purely mechanical concept whereas energy takes other non-mechanical forms I think that there is :D

Conservation applies in both cases, yes?

I had reduced my earlier question for brevity, but you took the reduced version literally by its word.
Let me reiterate the critical parts: " ... how does one fail where the other succeeds?" , or "the circumstances where momentum isn't conserved whilst energy is", or "Can you provide your description of the circumstances where momentum isn't conserved whilst energy is"

I want to see the justification for your original "isn't".
Despite the rather advanced physics, nothing you have posted thus far suggests, let alone actually proves, that unequivocal statement.





Let's recap how this discussion developed, just in case perspective has been somehow lost:

- Where is the relevance to QM, fields and relativity within your first mention of the issue?

- What exactly did you mean by "and road junctions would be very interesting places"?
Where is the "interesting physics" in this?

- And what have these newly raised theories got to do with cricket and snookerballs?
There was a strong implication that you disputed the momentum being conserved for those examples. If that wasn't the case, then what exactly was the point of raising them?

- And what of my unacknowledged rebuttal of your erroneous claim: "no corresponding movement in the opposite direction" ?

- And why even pose your "gedanken experiment" ? (which I fully addressed)

Why bring up those 100% "classical momentum conservation" examples when your answer supposedly lies wholly within the very non-classical?

I should repeat that the first mention of anything remotely relativistic, field-ish or QM-ish were subsequent to those 5 posts.
I have to say that those concepts are far too bleeding-edge to support something that was originally and consistently portrayed (those 5 posts) as being so straightforward, wouldn't you agree?



So, what gives, Dave? :scratchchin:

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PostPosted: Sun Feb 12, 2012 19:13 
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Consider this gedanken experiment. My system consist of a wooden cab mounted on four wheels on two solid axles. One of ther axles is connected via a gearbox to an electrical motor/generator machine whose terminals can be, through suitable control gear, connected to or disconnected from a secondary battery. As this is gedanken there is no friction in the mechanisms , no air resistance and no losses in the gearbox. The battery and charging gear are also perfectly lossless. The whole contraption weigh 1000kgm.

At the start of the experiment it is travelling at 10mS along a perfectly smooth surface and the electrical machine is not connected to the battery. Thus the momentum is, with suitable choice of axis, equal to 10^4 kg m/s and the kinetic energy is 5*10^4 joules.

At some point, by gedanken magic, the electrical machine is connected, as a generator, to the battery. The kinetic energy of the contraption is converted to electrical energy which is stored in the battery and it comes to rest. As the conversion process is perfect, energy has been conserved but Its momentum is now zero so momentum has not been conserved. The reason that momentum is not conserved is because the system is not closed and an external force has been applied through the wheels. It is easily shown that the value of that force is equal to the rate of change of momentum.

By further magic the battery is reconnected to the electrical machine, now acting as a motor, and the contraption accelerates until all the electrical energy is reconverted to kinetic energy when its speed will again be 10m/s.

Energy conserved in the system, momentum varied.

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PostPosted: Sun Feb 12, 2012 20:20 
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This is "doing my head in" ! Both seats of arguments make sense, (which clearly can't be the case!) so I'm watchign with interest.

One thing though, I can't see (maybe I'm not "gedankening" into the spirit of it it!) but I wondered about (instead of a generator and battery) a lovely frictionless flywheel and completely lossless transmission. Clearly the same experiment would work there, but the flywheel would have momentum (rotationally), which isn't your point - I see that. The question is whether the "movement" is the movement of lots of electrons at a particular velocity? (I haven't a clue, by the way, just interested...)


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PostPosted: Sun Feb 12, 2012 20:24 
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dcbwhaley wrote:
Consider this gedanken experiment. My system consist of a wooden cab mounted on four wheels on two solid axles. One of ther axles is connected via a gearbox to an electrical motor/generator machine whose terminals can be, through suitable control gear, connected to or disconnected from a secondary battery. As this is gedanken there is no friction in the mechanisms , no air resistance and no losses in the gearbox. The battery and charging gear are also perfectly lossless. The whole contraption weigh 1000kgm.

At the start of the experiment it is travelling at 10mS along a perfectly smooth surface and the electrical machine is not connected to the battery. Thus the momentum is, with suitable choice of axis, equal to 10^4 kg m/s and the kinetic energy is 5*10^4 joules.

At some point, by gedanken magic, the electrical machine is connected, as a generator, to the battery. The kinetic energy of the contraption is converted to electrical energy which is stored in the battery and it comes to rest. As the conversion process is perfect, energy has been conserved but Its momentum is now zero so momentum has not been conserved. The reason that momentum is not conserved is because the system is not closed and an external force has been applied through the wheels. It is easily shown that the value of that force is equal to the rate of change of momentum.

By further magic the battery is reconnected to the electrical machine, now acting as a motor, and the contraption accelerates until all the electrical energy is reconverted to kinetic energy when its speed will again be 10m/s.

Energy conserved in the system, momentum varied.

:lol: I can't believe the £60K physicist still doesn't get it :lol: It seems you are getting to be a bit forgetful too, Dave :roll:

You are as selective as Weepej, and that's really bad!
My previous post contained 12 separate and pertinent questions/requests from you; you ignored all of them :roll:

And now we have returned to the 100% classical examples. Where did your reliance on QM, relativity and fields go, Dave? :scratchchin:

And even more idiotic is that I have already answered your latest question; all is 100% answered here, Dave. :headbash:
In a direct response to that post, you said yourself:
DCB previously wrote:
"Of course. In those circumstances with no external force (i.e force from outside the closed system) applied momentum will be conserved. "
(the highlighting is mine)

Momentum is indeed conserved, it is just that you don't know how to close the system - still !!

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PostPosted: Sun Feb 12, 2012 20:35 
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Mole wrote:
This is "doing my head in" ! Both seats of arguments make sense, (which clearly can't be the case!) so I'm watchign with interest.

One thing though, I can't see (maybe I'm not "gedankening" into the spirit of it it!) but I wondered about (instead of a generator and battery) a lovely frictionless flywheel and completely lossless transmission. Clearly the same experiment would work there, but the flywheel would have momentum (rotationally), which isn't your point - I see that. The question is whether the "movement" is the movement of lots of electrons at a particular velocity? (I haven't a clue, by the way, just interested...)

Angular momentum is conserved separately from linear momentum (the latter is what has been scrutinised).

The creation of angular momentum will place a differential force on the object it is bound to; simply put: it pushes on one side of the object and pulls on the other (trying to spin it) - the net linear force on object is zero. This results with angular momentum of the object in the same plane but in the opposite direction, and no linear momentum.
Flywheel or electron flow, it makes no difference.

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PostPosted: Sun Feb 12, 2012 21:23 
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dcbwhaley wrote:
Consider this gedanken experiment. My system consist of a wooden cab mounted on four wheels on two solid axles. One of ther axles is connected via a gearbox to an electrical motor/generator machine whose terminals can be, through suitable control gear, connected to or disconnected from a secondary battery. As this is gedanken there is no friction in the mechanisms , no air resistance and no losses in the gearbox. The battery and charging gear are also perfectly lossless. The whole contraption weigh 1000kgm.

At the start of the experiment it is travelling at 10mS along a perfectly smooth surface and the electrical machine is not connected to the battery. Thus the momentum is, with suitable choice of axis, equal to 10^4 kg m/s and the kinetic energy is 5*10^4 joules.

At some point, by gedanken magic, the electrical machine is connected, as a generator, to the battery. The kinetic energy of the contraption is converted to electrical energy which is stored in the battery and it comes to rest. As the conversion process is perfect, energy has been conserved but Its momentum is now zero so momentum has not been conserved. The reason that momentum is not conserved is because the system is not closed and an external force has been applied through the wheels. It is easily shown that the value of that force is equal to the rate of change of momentum.

By further magic the battery is reconnected to the electrical machine, now acting as a motor, and the contraption accelerates until all the electrical energy is reconverted to kinetic energy when its speed will again be 10m/s.

Energy conserved in the system, momentum varied.




Hold up...... you've transferred your momentum outside the system by the forrce exerted on the roadway, presumeably the earth just got a little bit faster/slower. :drink2:


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PostPosted: Sun Feb 12, 2012 21:52 
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ed_m wrote:
Hold up...... you've transferred your momentum outside the system by the forrce exerted on the roadway, presumeably the earth just got a little bit faster/slower. :drink2:

According to DCB, road junctions are very "interesting places", yet we all know they are not - well everyone except DCB!

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PostPosted: Mon Feb 13, 2012 00:29 
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Steve wrote:
And now we have returned to the 100% classical examples. Where did your reliance on QM, relativity and fields go, Dave?


I didn't rely on Quantum Mechanics or relativity to bolster my argument. I introduced them as an interesting discussion point. Unfortunately you seem to engrossed in massaging your own ego to be interested in them.

Quote:
Momentum is indeed conserved, it is just that you don't know how to close the system - still !!


Of course I know how to close the system in order to conserve the momentum. But I was demonstrating that in a non closed system momentum is not conserved. That the rate of change of momentum is equal to the applied force. Right or wrong? And please don' t shout

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PostPosted: Mon Feb 13, 2012 00:36 
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ed_m wrote:
Hold up...... you've transferred your momentum outside the system by the forrce exerted on the roadway, presumeably the earth just got a little bit faster/slower. :drink2:


Exactly. I have transferred the motion out of the system by applying an external force to the system so that the systems momentum is not conserved. If I include the roadway in my system then the momentum is conserved. I was demonstrating what Steve can't get his head round. That momentum is only conserved in a closed system - one to which no external force is applied. I am not, as Steve thinks, denying that one can always close the system. Just that it is sometimes more convenient not to.

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Last edited by dcbwhaley on Mon Feb 13, 2012 02:10, edited 1 time in total.

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PostPosted: Mon Feb 13, 2012 00:40 
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Steve wrote:
According to DCB, road junctions are very "interesting places", yet we all know they are not - well everyone except DCB!


Of course there is no such thing as an uninteresting road junction any more than there is such a thing as an uninteresting number http://en.wikipedia.org/wiki/Interesting_number_paradox

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PostPosted: Mon Feb 13, 2012 00:42 
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dcbwhaley wrote:
... Of course I know how to close the system in order to conserve the momentum.
I think perhaps for the purposes of this detailed discussion that you show how you do, 'close the system'.
I am unclear (so far) if 'variable' is 'isnt conserved' too ?
I do find this a most interesting discussion, as I know others do too. I cannot say that I understand it to the levels that others clearly do, ... but to help me/us out, can I trouble you to reply to Steve's queries, as this can help me (and others) to better then appreciate your perspective.

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PostPosted: Mon Feb 13, 2012 00:48 
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dcbwhaley wrote:
.... denying that one can always close the system.
Can you 'close the system' since we all are on the planet? Surely the Earth has to be 'accounted for' albeit it perhaps minuscule if one is being precise? Or does one have to therefor specify that one has dismissed it - for sake of 'x' or 'y' ?

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PostPosted: Mon Feb 13, 2012 00:55 
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SafeSpeedv2 wrote:
Can you 'close the system' since we all are on the planet? Surely the Earth has to be 'accounted for' albeit it perhaps minuscule if one is being precise? Or does one have to therefor specify that one has dismissed it - for sake of 'x' or 'y' ?


Steve has made the point, with which I have no dispute, that when a car brakes it transfers its momentum to the earth and changes the rotational speed of the planet by a teensy-weensy bit. So if the "system" is car + planet, then momentum is conserved. but if the "system" is - as it might appear to the driver - only the car, then momentum is lost. But the conservation law is not violated because as external force has been applied so the conservation law is not valid.

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PostPosted: Mon Feb 13, 2012 01:01 
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SafeSpeedv2 wrote:
can I trouble you to reply to Steve's queries, as this can help me (and others) to better then appreciate your perspective.


No. Because they are not questions intended to bring enlightenment but rather to demonstrate how clever he is.
The fundamental law of physics states that if no external force acts on a closed system of objects, the momentum of the closed system remains constant. And the collarary to that is that if an external force does act then the momentum does change.

End of story. Except that the same is true of angular momentum hence this http://xkcd.com/162/

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