Re: Newton's 3rd law? was Re: inertial forces (definition)

[Leigh Palmer <palmer@SFU.CA>]

At 5:54 PM -0700 10/18/99, Cliff Parker wrote:
> Leigh Palmer wrote:
>
> > Today we understand that Newton's third law doesn't hold for
> > gravitational forces. (There is no body on which you exert a
> > force which is equal in magnitude and opposite in direction to
> > the force of gravity which acts on you, even on a nonspinning
> > planet.) One must inject the intermediary of a field (which is
> > not a body) to save a semblance the appearances.
>
> I am surprised by this statement.  Would you please expand on this idea?  I
> have been teaching my students that they attract the earth as the earth
> attracts them.  If I am teaching wrong ideas I would at least like to know
> why the idea is incorrect.  Are you saying that field theory replaced
> Newton's gravitational force theory, just as general relativity replaces
> both?  I am not thoroughly versed in field theory but have thought that it
> was an alternative to the idea of a gravitational force but not necessarily
> superior.  Can you briefly tell me why field theory is superior to Newton's
> theory?
>
> Cliff Parker

Last night I forgot to turn off my email machine at school and as
a consequence I missed all the action. My mail accumulated in my
office and I saw nothing at home as a result. David Bowman did a
lovely job of answering Cliff's question, but it was a response to
a bomb I had placed, and I was just a little disappointed to have
missed the chance to defuse it myself. However, David did a better
job of explaining me than I can, so thank you, David.

The idea that the reaction force is equal and opposite implicitly
entails the requirement of simultaneity, and of course we know
that simultaneity has no meaning for spatially separated events.
While Newton would not have stated it in that way, he was bothered
by the idea of action at a distance. The field view has it that
the reaction is against the field itself. That is what is meant by
"saving the appearances". By inventing a body which we call the
gravitational field we obtain a contact interaction which does not
suffer from the problem David explained. (I distinguish between
inertial forces and forces of contact.) A different problem arises
when one turns on a flashlight in space. It accelerates like a
very feeble rocket. Maxwell invented the electromagnetic field to
offset the flashlight's momentum gain. (Well, he didn't have this
problem in mind. The Brits of that era used torches, which confuse
the issue considerably; they still call their flashlights torches,
but they've adopted Maxwell's invention of field momentum.) Later
field theorists invented the "photons" with which high school
students seem to be more comfortable, though I can't imagine why.

In my treatment of introductory classical physics inertial forces
are those which have two properties (or non-properties). They have
strength proportional to the mass of each body in the system and
they act on every body, and they have no third law counterparts.
Gravity and centrifugal force are the names we give to the two of
these forces that are important in the Earth-based laboratory, but
there is no need to introduce those concepts in the beginning.
Simply saying that "g" pervades space will suffice, and that each
body of mass m in the laboratory experiences a force downward of
magnitude mg models that aspect of the laboratory very nicely. I
go farther than this, of course; I call mg the weight, something
that I expect a larger number of the people in this group will now
do also. This weight is just what my ideal scale will read when
an object is placed upon it in the laboratory.

Leigh