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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
----- Original Message -----
From: "LaMontagne, Bob" <RLAMONT@providence.edu>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Friday, August 03, 2007 10:59 PM
Subject: Re: [Phys-l] interaction
Nice example. Person A pulls on one end of the broomstick. The
boomstick exerts an equal and opposite force on Person A. This is one
pair of NTN 3 forces. Another pair is between Person B and the other
end of the broomstick. In this case no one would say that Person A and
Person B forms a NTN 3 pair (even though, as you stated, they
"interact" indirectly).
If we replace the broomstick with gravitational fields and the people
with the Sun and a planet, all of a sudden we say (in most general
physics texts) that the NTN pair is the Sun and the planet. I would
seem that my word "fuzzy" hardly begins to cover this inconsistency.
Also, we talk of the field producing a force on the object, but I have
not seen a claim in a basic text that the object exerts a force on the
field.
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.
Bob at PC
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