Iner t ia l motion. I t is possible to describe electromagnetism as well as gravitation as an inertial force? This should mean that the trajectory of a test particle of any charge q and mass m, subject only to gravitational and e.m. (electromagnetic) forces, should be universal independent of q and m a n d invariant ly defined in a basic geometry for space-time. We shall show here that this is indeed possible. Inertial mass equals gravitational mass; hence all test bodies fall at the same rate in a gravitational field. This well-known inertial character of the gravitational force culminated in Einstein 's geodesic motion equation