  In this letter, we discuss the extension of Feynman's derivation of the
equation of motion to the case of spinning particles. We show that a spinning
particle interacts only with the electromagnetic and gravitational fields. In
the absence of the electromagnetic interactions, we rederive Papapetrou's
equations for spinning particles in the background of the conformal gravity. We
also find that the effect of spin coupled to non-constant electromagnetic
fields leads to further corrections to the Lorentz force equations. Some
discussions of these results are given at the end.
