  The conservation laws of nonrelativistic and relativistic systems are
reviewed and some simple illustrations are provided for the restrictive nature
of the relativistic conservation law involving the center of energy compared to
the nonrelativistic conservation law for the center of restmass. Extension of
the nonrelativistic interaction of particles through a potential to a system
which is Lorentz-invariant through order v^2/c^2 is found to require new
velocity- and acceleration-dependent forces which are suggestive of field
theory where the no-interaction theorem of Currie, Jordan, and Sudershan does
not hold.
