Re: third law forces cancel?

[John Denker <jsd@AV8N.COM>]

Larry Smith wrote:
> 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?

There should be no difference mathematically or pedagogically.
I would hope the students would realize that
  k = -j  <==>  k + j = 0
is a tautology.  They normally learn that in 4th grade if not
sooner.

>  lest they get confused with the 2nd law.

There *is* a connection to the 2nd law.  They just have to
make the right connection.  The connection is
  0 = F_total = m_total a_total
which is the equation of motion for the CM of the (j+k)
combined system.