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Re: [Phys-L] 3rd law



On 06/25/2013 07:15 AM, Bruce Sherwood wrote:

In connection with the FCI there was some commentary on Newton's 3rd law.
I'll mention that I learned from Fred Reif a useful, nontraditional way of
approaching the subject. Point out the "reciprocity" of the gravitational
and electric force laws: m1m2 = m2m1 and q1q2 = q2q1. From this it is clear
that the magnitudes of the two forces must be the same. A further step is
to view the contact between the big truck and the little car at the atomic
level, where one expects the interatomic forces to obey reciprocity since
they are electric in nature. This approach gives a sense of mechanism that
is lacking from the convoluted English-language version of the 3rd law. [a]

I consider that a bad idea, for numerous reasons; see below.

The more fundamental view is that momentum must be conserved, so that the
changes in the momenta of the truck and car must have the same magnitude
and opposite sign. [b]

Approach [b] is all ways better than approach [a]. Conservation
of momentum is simple, elegant, and applicable without exception.
There is nothing "convoluted" about it.

In contrast, approach [a] suggests that the third law is a special
property of the electrostatic and gravitational force laws, and
might not apply to other phenomena.

Furthermore, conservation of momentum connects to other things we
know about conservation (including conservation of energy, conservation
of charge, et cetera) ... and also connects to other things we know
about momentum. As such, it provides an opportunity to use the
"spiral approach" to pedagogy, and an opportunity to emphasize the
unity, grandeur, and simplicity of physics.

In contrast, approach [a] provides no semblance of unity, grandeur,
or simplicity.

The concepts and the terminology for conservation and for momentum
are standard across all of physics and engineering.

In contrast, it is a terrible idea to use the word "reciprocity"
as a substitute for the proper name of the third law. It's
nonstandard jargon, and it conflicts with a different, long-
established meaning of the word /reciprocity/ in physics.

An interesting point is that magnetic forces do not
necessarily obey the reciprocity relationship.....

There is a larger point here, not restricted to magnetism.

[long, complicated, over-specialized story snipped]

The fundamental point is that the modern notion of conservation
works just as well -- and just as simply -- for N-way interactions,
including arbitrarily many particles and/or fields.

In contrast, yet another problem with "reciprocity" ... as well
as with the original 17th-century statement of the 3rd law ... is
that they paint a picture that works only for two-way interactions.

Momentum however is conserved: there is a
change in the momentum associated with the fields, and the total momentum
of the universe doesn't change.

Yes. That's a more modern, simple, elegant, powerful way to
describe what's going on.

OTOH it would be even better to state the law as a *local*
conservation law. Momentum is conserved right here, right now,
on a point-by-point basis (not on a universe-by-universe basis).

Conservation is not the same as constancy. It is a huge step
in the wrong direction to replace the *local* conservation law
with a much less useful global constancy law. The latter is
impossible to apply in practice. That is, if momentum were to
disappear from here and reappear somewhere else, far away, out
of sight, it would violate local conservation, but we would
have no way of knowing whether or not it violated global
constancy.

For details on all this,
http://www.av8n.com/physics/conservative-flow.htm