Relativistic force transformation
  Formulae relating one and the same force in two inertial frames of reference are derived directly from the Lorentz transformation of space and time coordinates and relativistic equation for the dynamic law of motion in three dimensions. We obtain firstly relativistic transformation for the velocity and acceleration of a particle. Then we substitute them in the relativistic dynamic equation and perform tedious algebraic manipulations. No recourse were made to "general rules for the transformation of 4-tensors". Formulae obtained were verified in electrodynamics. 
Valery P. Dmitriyev