The simple (trivial?) work-kinetic energy theorem some of us are
referring to in this discussion is nothing more or less than a
restatement of Newton's 2nd Law. Perhaps we are at fault for not
specifying the limitations of the theorem (we are talking about
particles, or, if rigid bodies, referring only to translational KE).
We don't pretend that it contains new information, only that it may
in some applications be more convenient to use than F = ma. And we
don't include couples because they contribute nothing to the net
force.
--
Martha Takats
Department of Physics and Astronomy
Ursinus College
Collegeville, PA 19426