Third Law Forces

[Jim Green <JMGreen@SISNA.COM>]

> The statement is in section 9.3 on Conservation of Momentum, subsection
> "Law of Conservation of Momentum," specifically in the middle of page 248.
> The discussion makes complete sense on its own.  I guess it is just a
> question of carefully instructing students when 3rd-law pairs cancel and
> when they don't; the pedagogy of this was my original question.

In my senility I don't understand this thread.  Please help me out if you will:

Third Law "pairs" ALWAYS "cancel."   That is 3rd Law pairs are always equal
and opposite so if you happen have occasion to add them, they add to zero
ALWAYS.  If your text doesn't agree with this, then throw it out.

The question is why would you want to add them?   You surely wouldn't if
you are dealing with the 2nd Law.  IE they would be either internal to the
system of two particles or one of them would not be applied to the system
composed of one of the particles etc etc.

So could somebody please tutor me either with regard to the physics or to
the purpose of this thread.

Jim


Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen