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PostPosted: Wed Feb 15, 2012 18:20 
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botach wrote:
dcbwhaley wrote:
This argument is now about semantics rather than Physics so I shall...
___________________________________________________________________


Thanks ,gents , before the GOOD name of SS gets besmirched .Thats what I suggrested several posts previous .
So SHAKE HANDS & call it a draw .

It's a draw. But my handsare a bit grubby with clea ning the hen shed.:-)

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PostPosted: Wed Feb 15, 2012 21:00 
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botach wrote:
dcbwhaley wrote:
This argument is now about semantics rather than Physics so I shall...
___________________________________________________________________


Thanks ,gents , before the GOOD name of SS gets besmirched .Thats what I suggrested several posts previous .
So SHAKE HANDS & call it a draw .

Nope. The questions have not been answered; all the subsequent ones have been ignored.

dcbwhaley wrote:
It's a draw.

Who are you trying to kid? You weren't even close!

You evaded one of the critical questions and skirted around another. For the third/forth you repeated your thought experiment which I had already shot down, and shot down again with the very damning:
Steve wrote:
Consider the following thought experiment. A small self-powered (battery, ICE) moving vehicle uses a normal friction brake to dissipate the kinetic energy via (at least partly) thermal radiation (just like our original example). If “conveniently” choosing the system to be the planet and the vehicle, momentum is conserved but energy is varied.
Thus I can now make the bold statement, relating to your original example, that “momentum is always conserved; energy isn’t.”, yes or no?

That's a tricky one for you, huh DCB ! :hello: Even your "convenience" won't get you out of this one! But I guess it is more convenient for you to just ignore it altogether, right? :roll:

Let me prove how it applies to your original claim:
dcbwhaley originally wrote:
And controlled stopping of a car is involves converting that kinetic energy into heat.

So where did that heat go? That "heat" has a component in the form of thermal radiation, which immediately leaves your "system" as you had defined it. Thus, by your own definition, energy wasn't conserved. You were wrong from the start!

So as you can see, whatever answer you gave for the "description of the circumstances where momentum isn't conserved whilst energy is?" has disappeared in a puff of logic. And that was with my first thought experiment I posed to you - and it was based wholly on your example.

In any correctly closed system, energy and momentum always remain constant. The definition I gave proved that your portrayed interpretation is wrong.


Given that you suddenly gave up, and that you misrepresented, lied and were hypocritical, its a bit difficult to conclude you aren't trolling.

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PostPosted: Wed Feb 15, 2012 23:35 
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Well, to add my 2c (without reading through all the preceding.... :D )

I think the problem is that people have a bit of a problem with the concept of energy. The difficulty is that energy can shift between differnt flavours. Thermal, chemical, electrical, radient, Nuclear Strong Force, whatever! it may be always conserved but for most people it can be a bit hard to keep track of what is happening.

Momentum however is different, it is ALWAYS conserved ABSOLUTLY!

There are no different "flavours" of Momentum! there is no "Thermal" momentum or "Chemical" momentum or "Acoustic" momentum. It is just Momentum! and it is ALWAYS conserved!

Indeed, thinking off the top of my (Admittedly slightly Pissed just at the moment :drink2: ) head, I reckon that ALL the fundimental laws of Physics can probabally be written down using the law of conservation on momentum as the basic tool, even all the esoteric quantum stuff!

Interestingly, I can neither remember (Nor find online) any sign of an "SI" unit for "Momentum"! Momentum doesnt seem to have a "Unit" with which to describe it! Perhaps this is some sort of recognition as to JUST how fundimental momentum (And its absolute conservation) actually is!!

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PostPosted: Thu Feb 16, 2012 00:05 
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I think Wiki provides a better explanation than I can :
Wiki wrote:
Momentum
From Wikipedia, the free encyclopedia
This article is about momentum in physics. For other uses, see Momentum (disambiguation).

....
Momentum
SI symbol: p
SI unit: kg • m/s
Conserved: yes
Derivations from other quantities: p = mv

p = γm0v
Title page of the 1st edition of Isaac Newton's Principia defining the laws of motion.

In classical mechanics, linear momentum or translational momentum (pl. momenta; SI unit kg•m/s, or, equivalently, N•s) is the product of the mass and velocity of an object:

\mathbf{p} = m\mathbf{v}.

Like velocity, linear momentum is a vector quantity, possessing a direction as well as a magnitude. Linear momentum is also a conserved quantity, meaning that if a closed system is not affected by external forces, its total linear momentum cannot change. Although originally expressed in Newton's second law, the conservation of linear momentum also holds in special relativity and, with appropriate definitions, a (generalized) linear momentum conservation law holds in electrodynamics, quantum mechanics, quantum field theory, and general relativity. In relativistic mechanics, non-relativistic linear momentum is further multiplied by the Lorentz factor.

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PostPosted: Thu Feb 16, 2012 00:13 
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SafeSpeedv2 wrote:
I think Wiki provides a better explanation than I can :

If I can pull out the relevant part:

Wiki wrote:
Momentum

Conserved: yes

No "if"s; no "but"s; no candy or nuts!

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PostPosted: Thu Feb 16, 2012 00:31 
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Ah well, every day is a school day.

But,

What does "P" stand for?? Is there a "Name"??

Actually I reckon I shall create a unit! I shall designate it as the "Whump"!

1 "Whump" shall be the momentum of 1Kg traveling at 1M/S! This is actually a pretty good unit since it is about what a reasonably fit bloke can achieve with a standard hammer without trying too hard! so it is a "Real world" unit that most people will be able to understand and use!

(I would have liked to have used "Maxwell" but that one is of course already taken.... :) Lets see who can guess why I might have liked to use "Maxwell" for this?? ;) :D )

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PostPosted: Thu Feb 16, 2012 00:49 
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Dusty wrote:
Lets see who can guess why I might have liked to use "Maxwell" for this?? ;) :D )


A silver hammer weighing 1Kg would be (approximately - so don't start quibbling) only 31mm square by 100mm long, you old Beatles fan, you. :D :D

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PostPosted: Thu Feb 16, 2012 03:16 
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Dusty wrote:
What does "P" stand for?? Is there a "Name"??
I found this - I believe it is correct :
Wiki wrote:
 ρf free charge density (not including bound charge) coulombs per cubic meter
ρ total charge density (including both free and bound charge) coulombs per cubic meter

Dusty wrote:
Actually I reckon I shall create a unit! I shall designate it as the "Whump"!

1 "Whump" shall be the momentum of 1Kg traveling at 1M/S! This is actually a pretty good unit since it is about what a reasonably fit bloke can achieve with a standard hammer without trying too hard! so it is a "Real world" unit that most people will be able to understand and use!
Wonderful ... was that 'liquid influenced' ? :wink:
Dusty wrote:
(I would have liked to have used "Maxwell" but that one is of course already taken.... :) Lets see who can guess why I might have liked to use "Maxwell" for this?? ;) :D )
How about this :
Wiki wrote:
The maxwell, abbreviated as Mx, is the compound derived CGS unit of magnetic flux. The unit was previously called a line. The unit name honours James Clerk Maxwell, who presented the unified theory of electromagnetism, and was established by the IEC in 1930.
1 maxwell = 1 gauss × cm2 = 10−8 weber

In a magnetic field of strength one gauss, one maxwell is the total flux across a surface of one square centimetre perpendicular to the field.

Is that what you meant ? :)

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PostPosted: Thu Feb 16, 2012 10:34 
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Dusty wrote:
What does "P" stand for?? Is there a "Name"??

P is the usual symbol for momentum which is measured in kg.m/s or Newton.seconds. Never occurred to me that it doesn't have a proper name but I don't think "whump" is right. Whump sounds more like a force or an impulse. I think a "whoosh" is better. If Steve could arbitrate this we could put it to the SI technical committee next week. :D :D

Claire has, naughtily, strayed into relativity with her p = γmv. The Greek letter (is it lambda?) is the Lorentz correction - named for Hendrick Lorentz (1853-1928), 1920 Nobel laureate - and is needed because of relativistic time dilation at high speeds. It is equal to 1/sqrt(1 -v^2/c^2) where c is the velocity of light. Without that correction the momentum of a closed system wouldn't be conserved at relativistic speeds - unthinkable!!!

Although it is usual to ignore the Lorentz correction in normal life it is worth calculating that at 60mph speed is reduced to approximately 59.999999999993333333333332592593mph - check my arithmetic - which could, technically, be the difference between a conviction for speeding and an aquittal :drive1:

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PostPosted: Thu Feb 16, 2012 12:43 
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dcbwhaley wrote:
Without that correction the momentum of a closed system wouldn't be conserved at relativistic speeds - unthinkable!!!

The same holds with energy: E = ymc^2. Yet again unthinkable!!!

Being as you have now crossed your own drawn line, how about addressing the thought experiment I posed to you? Or wi you instead draw another new line?

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PostPosted: Thu Feb 16, 2012 13:31 
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The line was drawn under the argument twixt thee and me and I would be breaking faith with Botach to cross it. But that doesn't preclude me from talking to Dusty or Claire about Physics.

I don't recognize E = lambda * m*c^2. It isn't kinetic energy because it doesn't have a v term. It looks like mass-energy equivalence but I didn't realise that there was a Lorentz facro in that.

No arguing, Steve, just seeking enlightenment from an expert.

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PostPosted: Thu Feb 16, 2012 14:30 
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dcbwhaley wrote:
The line was drawn under the argument twixt thee and me and I would be breaking faith with Botach to cross it. But that doesn't preclude me from talking to Dusty or Claire about Physics.

I guess it is more "convenient" to discuss these issues with those who aren't at your level.
Perhaps someone will pose the same thought experiment to you!

dcbwhaley wrote:
I don't recognize E = lambda * m*c^2.

That doesn't mean it doesn't exist:
Image

edited subsquequent to next post, to add:
dcbwhaley wrote:
It isn't kinetic energy because it doesn't have a v term.

There is no need for a v term. We are talking about energy, not kinetic energy specifically. There is no conservation law for kinetic energy (as you have said previously, and your 'conveniently bound' example plainly shows).

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PostPosted: Thu Feb 16, 2012 17:10 
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Steve wrote:
That doesn't mean it doesn't exist


No of course it doesn't. Thank you for your help. It is a privilege to have such a learned person and patient teacher to turn to. Takes me back to my undergraduate days with Prof Kahn and Doc Davies. :)

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PostPosted: Fri Feb 17, 2012 09:48 
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Quote:
I don't think "whump" is right. Whump sounds more like a force or an impulse.


Which of course it is, In defence of the "Whump". Impulse (Force x Time) is exactly the same as Momentum (Mass x Velocity) Although it is not really (AFAIAA) a concept much used outside rocketry, it IS how a hammer acts as a "Momentum capacitor" and enables a puny Human to extert forces in the 10's (possibly even into the hundreds in exceptional cases) of tons of force using a small lump of metel. :D

F(1)t(1)=F(2)t(2)

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PostPosted: Fri Feb 17, 2012 22:16 
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Dusty wrote:
Which of course it is, In defence of the "Whump". Impulse (Force x Time) is exactly the same as Momentum (Mass x Velocity)


But that would mean that applying an impulse to a particle could change the particle's momentum. Which must be wrong since momentum is always conserved.

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PostPosted: Fri Feb 17, 2012 23:02 
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Quote:
But that would mean that applying an impulse to a particle could change the particle's momentum.


Which is exactly what it does do! I am a little confused here...

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PostPosted: Sat Feb 18, 2012 10:49 
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Dusty wrote:
Which is exactly what it does do! I am a little confused here...


So am I. Momentum must be conserved but, if you follow through with the hammer blow so that the hammer doesn't slow down, where does the momentum come from?

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PostPosted: Sat Feb 18, 2012 14:10 
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dcbwhaley wrote:
Dusty wrote:
Which is exactly what it does do! I am a little confused here...


So am I. Momentum must be conserved but, if you follow through with the hammer blow so that the hammer doesn't slow down, where does the momentum come from?


Are you being deliberately obtuse?

1. Only a small part of the hammer's momentum would be transferred so of course it would continue moving even if only minutely slower.

2. If a force is applied to the hammer to accelerate it back to the same pre-impact speed then then ultimately momentum will be conserved via that force.

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PostPosted: Sat Feb 18, 2012 15:06 
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Dusty wrote:
Quote:
I don't think "whump" is right. Whump sounds more like a force or an impulse.


Which of course it is, In defence of the "Whump". Impulse (Force x Time) is exactly the same as Momentum (Mass x Velocity) Although it is not really (AFAIAA) a concept much used outside rocketry, it IS how a hammer acts as a "Momentum capacitor" and enables a puny Human to extert forces in the 10's (possibly even into the hundreds in exceptional cases) of tons of force using a small lump of metel. :D

F(1)t(1)=F(2)t(2)

Very simplistically (probably should integrate):
We know F = ma
Multiply both sides by t: Ft = mat
We know v = at + u (simplistically: velocity = acceleration x time)
substitute: Ft = mv = ρ

Like I said, it is a bit simplistic.
As I said in an earlier post, there must be equal and opposing forces, over the time duration of when the force is applied, so yes, I do completely agree with you: the Fts are conserved.

dcbwhaley wrote:
But that would mean that applying an impulse to a particle could change the particle's momentum. Which must be wrong since momentum is always conserved.

When applying Ft: where does the F come from?

dcbwhaley wrote:
So am I. Momentum must be conserved but, if you follow through with the hammer blow so that the hammer doesn't slow down, where does the momentum come from?

Pardon?
Could you explain that, please?
:D
Here we go again :D

PS: the hammer always slows during the impact (F=ma), but it need not have slowed by a noticeable amount and/or you re-accelerated it by trying to follow through. edit: like wot Toltec said.


Toltec wrote:
dcbwhaley wrote:
So am I. Momentum must be conserved but, if you follow through with the hammer blow so that the hammer doesn't slow down, where does the momentum come from?

Are you being deliberately obtuse?
Steve previously wrote:
dcbwhaley previously wrote:
But if a rocket from Ursa Major were to smash into the side of the car it would move rapidly in the direction of Polaris with no corresponding movement in the opposite direction

How utterly stooopid!

I've just realised that lightning has 'struck twice' :(

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PostPosted: Sat Feb 18, 2012 19:39 
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Sorry folks. Too bust working on my perpetual motion machine to get involved in this discussion again.

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