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Jack,
You and I had an off-list discussion of this the last time this subject came up. Now I find a neat animated illustration of the very example which I then suggested to you as a clear violation of N3 when only the mechanical momenta (mv) of the particles are considered. Please go to:
http://jlnlabs.imars.com/lifters/lorentz/index.htm
Also consult the Feynman Vol II references.
Bob Sciamanda
Physics, Edinboro Univ of PA (Emeritus)
www.winbeam.com/~trebor
trebor@winbeam.com
----- Original Message ----- From: "Jack Uretsky" <jlu@hep.anl.gov>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Saturday, August 04, 2007 5:50 PM
Subject: Re: [Phys-l] interaction
| Hi Bob-
| I don't understand "the linear momentum is not conserved" (unless
| you're including brehmstrahlung). Have I forgotten something?
| Regards,
| Jack
|
|
| On Sat, 4 Aug 2007, Bob Sciamanda wrote:
|
| > Bob LaMontagne writes:
| > "I feel that some of the suggestions posed so far, like not using NTN
| > 3 for fields, sweep the issue under the rug. Perhaps I'm just making a
| > mountain out of a molehill here, but all of this is part of
| > introductory physics courses and it seems that there must be a better
| > way to handle the terminology."
| > ************
| > So far this thread seems to have considered fields modeling only
| > Newtonian gravitation action at a distance. N3 is significantly more
| > stressed if one considers electromagnetic interactions:
| >
| > If you consider two electrons swerving past each other, the Lorentz
| > particle forces
| > are not in general equal and opposite, and the linear momentum of the
| > bare particles
| > is not conserved. We ascribe momentum and energy to the accompanying
| > fields so that the overall momentum and energy of the system are
| > conserved. This is a
| > testable model and works. (Eg. the fields can transfer real
| > "particle"
| > momentum/energy to distant objects.)
| >
| > Ascribing energy and momentum to the fields makes the phenomena
| > tractable, as opposed to the impossible task of accounting for every
| > (however distant) particle in the universe which might be affected by
| > the fields of the two electrons of interest (not to mention the
| > problem of extent in time of these effects). It is indeed remarkable
| > that including the role of the fields allows us to treat the problem
| > as if it were an isolated, localized sytem and ignore the rest of the
| > universe, for a wide scope of situations.
| >
| > The bottom line is that the system of two interacting electrons is not
| > a
| > simple two-particle system . . . there are other entities involved -
| > accountable by the
| > fields - whose momentum/energy must be recognized.
| >
| > N3 envisions only interactions between two, otherwise isolated bodies.
| > N3
| > is at most clumsy in multi-entity interactions. The more fundamental
| > model of momentum conservation applies to all these situations - N3 is
| > a
| > corollary for the special case of an otherwise isolated two-body
| > interaction.
| >
| > N3 was an ingenious insight of Newton into a basis for the
| > impossibility
| > of certain phenomena (eg., self acceleration). I always include in
| > teaching N3 an open discussion
| > about impossible feats which we could accomplish if N3 (more
| > generally,
| > conservation of momentum) were not true.
| >
| > Bob Sciamanda
| > Physics, Edinboro Univ of PA (Emeritus)
| > www.winbeam.com/~trebor
| > trebor@winbeam.com
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