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

[Leigh Palmer <palmer@SFU.CA>]

Paul Johnson wrote:

> Leigh Palmer wrote:
>
> > The force that acts on a body in an Earth based lab, that I have been
> > calling
> > "weight", doesn't have a
> > third law partner. The gravitational force the body exerts on the Earth
> > isn't
> > quite the same magnitude and it doesn't act in exactly in the opposite
> > direction. Why tell your students it does?
>
> Are you saying, Leigh, that the 3rd law holds only for objects that are in an
> inertial reference frame? To be sure, the force that Earth exerts on me is
> slightly greater in magnitude than the force I exert on Earth because I am
> not in
> equilibrium -- I am accelerating toward Earth's axis. But wouldn't the two
> forces
> have equal magnitudes if I were in equilibrium?

Yes. If the Earth were not rotating (or revolving about the Sun) and
if only you and the earth constituted the universe, and the Earth and
you were both spherically symmetrical (or at least you were standing
upright), and you weren't moving, then the third law would work fine
by considering just the force on you due to the earth and the force
on the earth due to you. In my view it is not necessary to make all
those exceptions (and some others I may have missed) to teach
elementary physics. The idea that a field pervades the space in the
laboratory in the mundane rotating, revolving, etc. allows us to
ignore all that stuff outside the laboratory and concentrate on the
local problems at hand.

> As for what to tell my students, I always start the course by describing how
> physicists must begin their descriptions of nature with the simplest possible
> model (I call it the ideal world model) because the real world is too
> complicated
> to describe accurately. When we cover the laws of motion, I use a simple
> model in
> which the two 3rd law forces are equal and opposite because at that point
> in the
> semester we are considering Earth to be at rest.

My point is that you don't have to mention the details of the world
outside the laboratory *at all*. It is brilliantly evident (to me, at
least) that this is a simpler model than one which invokes the small
vertical displacement approximation to the inverse square force of
gravity.

> Several weeks later, when we get into angular motion, I complicate this simple
> model of object-Earth attraction by including the effects of centripetal
> acceleration. We discuss many examples of the forces acting on an object
> that is
> in circular motion, such as Earth's gravity and the seat both exerting
> downward
> forces on your body at the top of the vertical-loop carnival ride. Somehow, we
> never get into the issue of the 3rd law being violated, nor do I believe my
> students are any worse off with this incremental approach.

I'm sure your approach will work; I was exposed to it and I learned
physics, I think, despite it. I have seen too many misconceived
interpretations of that approach to be entirely happy with it. That
is why I advocate this *simpler and more correct* approach to the
task of teaching physics. I think that after seeing so many free
body diagrams with gratuitous "centripetal forces" included for no
conceivable reason other than the fact that a physics teacher or a
textbook told the student it must be there, I've decided to try to
do things differently. I'm not heartened by what I hear in this
group. I won't try to write my own physics textbook because it seems
to me that there is a deep commitment to tradition that would likely
prevent its getting past the review stage because it would find no
market. It is disappointing to see such strong reaction forces in a
discipline that has been, at its best, wildly nontraditional.

Leigh