Re: Apparent weight

[Leigh Palmer <palmer@sfu.ca>]

> Hmmm... Since you can't tell if you're in an accelerated frame of reference
> of in a local inertial frame with a gravitational field Newton's Third Law
> should never apply and Momentum should never be conserved (except by
> accident of course).

You will find that conservation of momentum is a principle that works
quite well in the horizontal plane in an accelerated frame. There is no
need to give it up. I'm very curious about how you teach your students
the first law of motion. I start by showing my students that we don't
live in an inertial frame. The first law is not trivial by any means;
it is a great intellectual feat to abstract to an environment one has
never experienced for any appreciable time - and lived to tell of it!

> I really believe that working with "inertial forces" with introductory
> students leads to more problems than it solves.  I prefer to introduce
> "inertial forces" when needing to transfer to an accelerated
> frame of reference to obtain "pure rotational motion" so I can use
> tau=I*alpha which only holds under that condition and the equivalence
> principle later in the course as something that Einstein formalized.

I don't refer to "inertial forces". I treat them transparently, since I
can't distinguish them locally from gravitational forces. That's even
simpler.

> As a separate issue, I can only derive the relation tau-net-on=dL/dt (all
> vectors of course) for macroscopic objects from the similar equation for
> individual particles by assuming that the internal forces that hold bodies
> together are central forces.  Does that mean that objects that are held
> together by magnetic forces (is this even possible?) must radiate angular
> momentum (true, too in the form of E&M waves) and that's built into
> Classical Mechanics?

Hmm... I've got some rare earth magnets I should show you.

Magnets don't radiate angular momentum (presumably in the form of
photons). Classically they should, of course, because they produce
their fields by circulating net currents, necessitating the
acceleration of charges. Bohr was among the first to suggest that
somehow this sort of circulation did not always imply radiation,
even though classical electrodynamics says it does.

Leigh