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

[Bob Sciamanda <trebor@VELOCITY.NET>]

----- 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.

Neglecting acceleration, radiation and time retardation effects:

The Coulomb electrostatic forces are equal and opposite - no problem.  But
the Bio-Savart/Ampere "magnetostatic" forces must give pause:

A bit of quick "right-hand-ruling" will show that proton A produces zero
magnetic field at the location of proton B;  but proton B's magnetic field
at proton A's location is non zero and is in fact perpendicular to proton
A's velocity. Thus B exerts significant magnetic force on A, while A
exerts zero magnetic force on B (at this instant), neglecting the
enumerated complications.

Newton knew nothing about velocity dependent forces.  He would have
welcomed them as making possible an operational meaning of "absolute
velocity".  Einstein realized this and it was his revulsion for this
absolute velocity notion that drove him to SR as an interpretation of the
Lorentz force.

In the meantime, the Maxwellian-Newtonians pointed out that Isaac's 3rd
law explicitly referred to the interaction of TWO bodies and required
equal and opposite forces (Check it out).  The proposed two proton system
is not an isolated two body system - these charges will affect not only
each other but, in principle, every other charged particle in the
universe.  This is not a two body problem.  The calculation can be
localized by appropriately endowing the local fields with momentum and
energy.  The 3rd law is irrelevant (more than two objects are
interacting), and momentum can be conserved.

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor