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

----- Original Message -----
From: Leigh Palmer <palmer@SFU.CA>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, October 20, 1999 10:40 PM
Subject: Re: Newton's 3rd law? was Re: inertial forces (definition)
. . .

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?
. . .
Leigh

I would say that the gravitational force pair between the earth and a
surface object is a valid third law pair. It is the fictitious
centrifugal force on the surface object which has no agent and so is not
part of a third law pair. This centrifugal effect is observed because
the observer is accelerating. (All of this is said within Newtonian
mechanics)

The gravitational force will participate in momentum conservation; the
centrifugal force will not.

(This is also a motivation for not including inertial forces in the
concept of "weight".

Bob

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