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

[Leigh Palmer <palmer@SFU.CA>]

> At 7:40 PM -0700 10/20/99, 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?
>
> so we shouldn't be using an analogy between the 'force' 'caused' by a
> gravitational field acting on a mass and the electrostatic force on a
> charge Q in an electric field E?
>
> does this 'force' have the same pedagogical problem in your view?

No. Pedagogically the electrostatic force can be introduced into this
system *ad hoc*. That's the way it is normally introduced anyway. No
problems arise antil accelerated charges begin to radiate. Even the
motion of charges is accounted for *ad hoc* by introducing magnetism,
though it is to my taste to let the students in on a good bit of the
relativity behind magnetism. They've heard about Lorentz contraction
in high school and probably even know a formula or two. It makes the
magnetism just a little less *ad hoc*.

Leigh