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Re: Newton's 3rd Law

Nothing is intrinsically wrong with action/reaction as far as I know.
However, it has unfortunately become a sort of cultural artifact, a
bit of jargon that everybody has heard and remembered and that tends
to substitute for intuitive understanding. By singing the
action/reaction line, and thereby stimulating all the associated
memories (some of which are wrong), a comprehension of the deep
meaning of the third law is impeded (or at least that is my
experience). What has been hidden is: what is meant by "action" and
"reaction"? On what objects do they act or react? It seems to avoid a
deal of confusion to always explicitly identify two separate forces
acting on two separate objects, just as you would do in drawing a
free body diagram. Requiring reasoning in terms of "force on object A
by object B" and vice versa, rather than compressing these ideas into
single terms, is beneficial and there is a great deal of
non-anecdotal evidence to support this (see e.g. Arons' book).

What's wrong with the notion of action/reaction? By using concrete
examples to help the student experience Newton's 3rd Law, it seems quite
natural to describe these as action/reaction, and, consequently, we find
that forces aways occur in pairs.


Now get them to explain it to you and kill that action/reaction
thing in the egg. To me, Newton's laws are nothing more than an
operational definition of the word "force." The first two laws tell
you what a force does -- causes an acceleration -- and the third law
tells you that really there are no forces. There are _interactions_
between particles and every interaction necessarily involves a _pair_
of forces -- each particle interacts with the other one. It is just a
way of saying that in an interaction, neither particle is priviledged
in any way.

Paul J. Camp

----------
TK McCarthy Email:mcca6300@spacelink.msfc.nasa.gov



Paul J. Camp "The Beauty of the Universe
Assistant Professor of Physics consists not only of unity
Coastal Carolina University in variety but also of
Conway, SC 29526 variety in unity.
pjcamp@csd1.coastal.edu --Umberto Eco
(803)349-2227 The Name of the Rose
fax: (803)349-2926