  It is shown that if we can define a physical quantity with proper character
in a given inertial reference frame (kinematic, dynamic, electromagnetic in its
nature) which transforms when detected from a reference frame relative to which
it moves with velocity $u_x$ as $F=\f{F^o}{\sqrt{1-\f{u_x^2}{c^2}}}$ then we
can derive for it transformation equations following one and the same
procedure, which involves the addition law of relativistic velocities which can
be derived without using the Lorentz transformations. The transformation
equation derived that way, generates the physical quantities $u_xF$ and
$u_x'F'$, for which physicists invent names reflecting theirs physical meaning.
