An electric charge Q of mass m, in the form of a spherical shell of radius a, moving at time t with velocity v and acceleration d v / d t, generates an electro-dynamic field X proportional to the acceleration. The field X acts on the self-same charge Q to produce an inertial force Q X = – m (d v/ d t), equal and opposite to the accelerating force, in accordance with Newton?s 2 nd and 3 rd laws of motion. This explains the origin of inertia as electrical and internal to a body, contrary to general relativity. An expression deduced for the mass m as a constant, in terms of Q and radius a, is compared with the electrostatic energy E of the charge to obtain E = ? mc 2, in contrast to the mass-energy formula of special relativity, where c the speed of light in a vacuum.

Keywords: Acceleration, electric charge, field, force, inertia, velocity.