> Doesn't N3 tell us that for every centripetal
> force on an object there is an equal
> centrifugal force on another object?
Yes, on ANOTHER object. In the example that
was discussed yesterday the track (object #2)
acts on the sliding thing (object #1) with a force
directed toward the center of rotation. We were
asked to construct a free-body force diagram
for the object #1. In that context, the equal and
opposite force, acting on the object #2, was
not relevant. One may be tempted to call the
force acting on the object #2 "centrifugal" but
that term has already been reserved for
something very different.
Ludwik Kowalski