  This paper describes a curious phenomenon: a particle in a rapidly varying
potential is subject to an effective magnetic-like force. This force is in
addition to the well-known ponderomotive force, but it has not been shown to
exist before except for the linear case of a rapidly rotating quadratic saddle
potential. We show that this is a universal phenomenon: the magnetic-like force
arises generically in potential force fields with rapid periodic time
dependence, including but not limited to rotational dependence.
