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

[Leigh Palmer <palmer@SFU.CA>]

Bob Sciamanda says:

> ----- Original Message -----
> From: Leigh Palmer <palmer@SFU.CA>
> To: <PHYS-L@lists.nau.edu>
> Sent: Thursday, October 21, 1999 1:35 AM
> Subject: Re: Newton's 3rd law? was Re: inertial forces (definition)
>
>
> > > 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,
> > . . .
> > Leigh
>
> Pedagogically, I think it needs to be pointed out that even neglecting
> acceleration, radiation and time delay, the Lorentz (electromagnetic)
> force offers an anomaly for Isaac's 3rd law:
>
> Consider two interacting protons.  Proton A is (at this instant) moving
> directly toward proton B.  The (instantaneous) velocity of proton B is
> perpendicular to the velocity of proton A.

I know Bob is trying to be helpful here, but somehow I feel that my
point was utterly lost in all this scuffling, so I must reenter the
fray, at least briefly. My thanks to John Mallinckrodt for helping out
in the fight today. I had missed him.

Let me be very clear about what I was discussing. I wanted to make the
simplest model possible which would describe the physics of the
Earthbound laboratory in an introductory physics course. I had no need
to discuss such arcana (from the introductory student's point of view)
as inverse square gravitational force and centrifugal force to
accomplish this task; all I needed to do was to prescribe a physically
respectable way to account for the mechanism that imposes upon every
body in the laboratory a force proportional to its mass, which I call
"weight". My weight differs from the traditional definition, which
many here support, but the relative merit of which utterly evades my
understanding (except, maybe, that Tevye would have liked it).

I was asked how I would introduce the electrostatic field into my
model; I answered the question. Now a new clarification of the
pedagogy shows up, the introduction of the Lorentz force. I do not
think it is necessary to point this out at the time I introduce the
electrostatic force. Similarly, I do not think it is necessary to
introduce students to the principle of equivalence when I construct my
model of the Earthbound laboratory. I merely note in passing that my
model is *better* than the traditional model, and I adduce its evident
consistency with the principle of equivalence to support my viewpoint.
Somehow this seems to have made people think I'm introducing GR at the
elementary level and I'm not doing that; as I must repeatedly protest,
I don't even know GR!

Leigh