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

[Leigh Palmer <palmer@SFU.CA>]

> > Later
> > field theorists invented the "photons" with which high school
> > students seem to be more comfortable, though I can't imagine why.
>
> Come on Leigh, marbles I understand invisible fields made of ???? now those
> things are a little strange.

I grant you your point, but I am uncomfortable with photons. They
are frequently invoked when there is no good reason to do so. The
photon is a quantum mechanical entity. It is certainly no less
unreal than the electromagnetic field. Unless there is a quantum
aspect to a problem I prefer a classical approach to solution.

> > In my treatment of introductory classical physics inertial forces
> > are those which have (TWO) properties (or non-properties). (1)They have
> > strength proportional to the mass of each body in the system and
> > (2)they act on every body, and (3)they have no third law counterparts.
>
> I have heard there are three kinds of physicists those who can count and those
> who can't.  ; )

I meant for you to parse that with the comma. Substitute
"(1)They act on every body in the system with a strength
proportional to the mass of the body, and (3)they have no
third law counterparts." Oops! I still can't count!

> Seriously, I am having trouble with the idea of inertial force.  The current
> discussion on that topic has not helped me to understand.  Perhaps I need
> to go
> back and read more carefully.  Can you give some more detailed examples?  I
> think that I understand how centrifugal force can be called an inertial force
> (it is caused by an objects inertia and would have no third law counterpart).
> But what about gravity?  I see where the gravitational force would fit #1 and
> #2, but what is this about #3.  Are you saying that the force of gravity
> has no
> gravitational counterpart because it acts at a distance?  If this is the case
> then as what force would not act at a distance?  Didn't you say that if
> the idea
> of a field is used the action at a distance problem is eliminate?

The task at hand in teaching elementary physics, as I see it, is
to describe the mundane first. I want a mathematical model of the
physics laboratory on Earth's surface which will give an accurate
prediction when applied to such a system. I do this by ascribing
to each object in the system a weight. The weight is an inertial
force. It is a vector, but the direction is downward in all cases,
so the distinction from a scalar is trivial; I don't mind if you
want to call weight a scalar. I do mind if you want to call weight
the gravitational force. There's no justification for that when
you look around *inside* the laboratory, and besides, with outside
knowledge we know that isn't even right; the weight includes a
centrifugal component that registers on a scale even if you don't
believe in it. From experiments done inside the laboratory you
can't tell if you are accelerating in a rocket ship, or at rest on
Earth's surface, or as John Mallinckrodt points out, hovering in
a rocket ship one meter above a launchpad on Earth. Inside the
laboratory all situations lead to the same physical results for
experiments.

Later on you can build a model which attributes weight to gravity,
and then correct it by introducing centrifugal force. There is no
reason to get into such details doing Newtonian physics in the
Earthbound laboratory.

As I said before, my model distinguishes forces as forces of
contact and weight, which I do not call "inertial force" until
much, much later, and probably never in a "noncalculus" course
like the one I'm teaching now. I won't even get into centrifugal
force, except mentioning it in passing. The fact that there is no
reality to the designation "force of contact" notwitstanding, the
model works well. I have to add a new kind of force when I
introduce electrostatics of charged bodies, and if they move fast
enough I may have to get into complications. But I'm not going
even as far as electrostatics in this course.

Leigh