EQUATIONS OF MOTION FOR SPINNING PARTICLES IN EXTERNAL ELECTROMAGNETIC AND GRAVITATIONAL-FIELDS

The equations of motion for the position and spin of a classical particle coupled to an external electromagnetic and gravitational potential are derived from an action principle. The constraints ensuring a correct number of independent spin components are automatically satisfied. In general the spin is not Fermi-Walker transported nor does the position follow a geodesic, although the deviations are small for most situations.