Suppose I have two charges of mass m'_c stationary in S' separated by a constant distance and one of the charges is released at time t' and accelerates with acceleration _a' to a velocity du' a time dt' later. The force is then just m'_c*du'/dt'. If I transform this event to frame S, then the velocity is transformed using the addition of velocities formula and dt' is transformed using the LT giving: du = (V + du')/(1 + Vdu'/c^2) - V dt = gamma(dt' + dx'V/c^2) = gamma(dt' + 1/2 _a dt'^2 V/c^2) = gamma dt'. So the acceleration in S is: ((V + du')/(1 + Vdu'/c^2) - V)/gamma Is this correct so far? I need to transform m'_c to m_c to work out the equivalent force in S causing the moving charge to accelerate. How do I do this? How do I work out the force on a stationary charge in S placed at the point where the accelerating charge is? Thanks for your interest.