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

[paul o johnson <pojhome@FLASH.NET>]

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?
Are you saying, Leigh, that the 3rd law holds only for objects that are
in an inertial reference frame? To be sure, the force that Earth exerts
on me is slightly greater in magnitude than the force I exert on Earth
because I am not in equilibrium -- I am accelerating toward Earth's axis.
But wouldn't the two forces have equal magnitudes if I were in equilibrium?

As for what to tell my students, I always start the course by describing how physicists must begin their descriptions of nature with the simplest possible model (I call it the ideal world model) because the real world is too complicated to describe accurately. When we cover the laws of motion, I use a simple model in which the two 3rd law forces are equal and opposite because at that point in the semester we are considering Earth to be at rest.

Several weeks later, when we get into angular motion, I complicate this simple model of object-Earth attraction by including the effects of centripetal acceleration. We discuss many examples of the forces acting on an object that is in circular motion, such as Earth's gravity and the seat both exerting downward forces on your body at the top of the vertical-loop carnival ride. Somehow, we never get into the issue of the 3rd law being violated, nor do I believe my students are any worse off with this incremental approach.

poj