Re: third law forces cancel?

[Muhsin Ogretme <ogretmem@BOUN.EDU.TR>]

Larry Smith wrote:

> Many authors say that N's 3rd law pair forces do not cancel because they
> are acting on different (i.e. opposite) objects.  Randy Knight, in his new
> PER-based textbook, says, in his discussion of conservation of momentum,
> that
> \vec{F}_{k on j} + \vec{F}_{j on k} = \vec{0}.  Is the discrepancy between
> these two statements real or imagined?  Would you tell your students one or
> both or neither of these statements?  I have told students that we don't
> even add forces on different objects together; should I not have?  What's
> the best pedagogical approach to this?  Further comments?
>
> Thanks in advance,
> Larry
You can add any two vector having same magnitude and opposite direction
and find a zero vector, there is no problem with it. However it has no
physical meaning unless they are -lets say- force vectros acting on the
SAME object. Newton's 3rd law force pairs DONOT act on the same object,
consequently DONOT create a combined effect on the object.