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

[Leigh Palmer <palmer@SFU.CA>]

> In a message dated 10/18/99 7:53:38 PM Central Daylight Time,
> cparker@EMPOWERING.COM writes:
>
> > I am surprised by this statement.  Would you please expand on this idea?  I
> >  have been teaching my students that they attract the earth as the earth
> >  attracts them.  If I am teaching wrong ideas I would at least like to know
> >  why the idea is incorrect.
>
> Cliff,
>
> Physics is the study of nature.  If you teach your students that they attract
> the earth as the earth attracts them, then you are in good company.  And I
> quote from texts concerning their presentation of gravitation:

That's good as far as it goes, but 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?

> Saying this, I concede that this model is ultimately wrong, as all our models
> of nature are probably going to be wrong.  At least I hope this is true.  I
> certainly hope we never reach the point when everything is predictable.

Physics doesn't pretend to make models that describe Nature perfectly.
Some models are better than other models, but the lesser models may be
adequate for some tasks. My problem is using a model that introduces
complications with no consequent benefit. This model does that. There
is no consequence of which I am aware of the fact that I exert a force
on the Earth. The existence of that force is inconsequential. It is a
perfect example of an item that can be put on a multiple choice exam
but which has very little other purpose.

Leigh