Re: third law forces cancel?

[John Clement <clement@HAL-PC.ORG>]

A number of others responded quite correctly, but none commented on one of
the real problems.

The third law involves a very subtle idea that is often misapplied even by
advanced students.  It involves some big misconceptions.  Students
immediately latch onto the idea of the forces as being equal, but fail to
apply it in all situations.  In addition they use the idea of equality to
identify third law pairs.  Hence they identify the normal force and the
weight as third law pairs on a stationary object.

Students need to identify third law pairs as being opposite, on different
interacting objects, and of the same kind, and as having switched objects
and agents.

Now Knight's objective may be OK, in the context of momentum, but it is
difficult to tell in a snippet.  The statement should be judged in context.
What page is it on, Chapter&section?

John M. Clement
Houston, TX


> 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