Re: third law forces cancel?

[Robert Cohen <Robert.Cohen@PO-BOX.ESU.EDU>]

Are you saying that N3L *cannot* be expressed mathematically as
 \vec{F}_{k on j} = - \vec{F}_{j on k}
or *should* not?  Or, are you saying that pedagogically we should avoid
including "internal" forces when writing equations for the "system"?

____________________________________________________
Robert Cohen; 570-422-3428; www.esu.edu/~bbq
East Stroudsburg University; E. Stroudsburg, PA 18301


> -----Original Message-----
> From: Forum for Physics Educators
> [mailto:PHYS-L@list1.ucc.nau.edu] On Behalf Of Larry Smith
> Sent: Monday, October 11, 2004 12:35 PM
> To: PHYS-L@LISTS.NAU.EDU
> Subject: third law forces cancel?
>
>
> 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