  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.
