Re: third law forces cancel?

[Larry Smith <larry.smith@SNOW.EDU>]

At 12:49 PM -0400 10/11/04, Robert Cohen wrote:
> 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"?

Yeah, kinda the latter.

I'm wondering if pedagogically
\vec{F}_{k on j} = - \vec{F}_{j on k}
is not the same thing as
\vec{F}_{k on j} + \vec{F}_{j on k} = \vec{0}; sure, mathematically they
are the same thing, but should we tell students the 3rd law pair forces are
equal in magnitude but opposite in direction, or should we tell them they
sum to zero?  I think there may be a difference pedagogically, lest they
get confused with the 2nd law.

Larry