Some properties of motion equations in General Relativity for a particle under the influence of external forces in addition to the gravitation are studied. The geometrical interpretation of many results is discussed. In particular it is shown that the external forces are acting in such a way that a particle describes in the space time variety, instead of a geodesics, a curve for which the geodetical curvature vector is given by the external force acting on the unitary mass and along which the velocity vector undergoes a Fermi transport. The results obtained are applied to the study of the behaviour of a charged particle moving in a uniform gravitational field and under the action of an electrical field due to a charge at rest in a point of the space.