  Transformation equations for the kinetic energy of a tardyon are derived in
the limits of classical and of special relativity theory. Two formulas are
presented. In the first one the energy of the particle in one of the involved
reference frames is presented as a function of its proper energy, of the
relative velocity of the two frames and of its speed in the second one. In the
second one the kinetic energy in one of the involved reference frames is
expressed as a function of its kinetic energy in the second one of its proper
energy, of the relative velocity of the involved inertial reference frames and
of its velocity relative to that frame. The obtained results are extended to
the case of a photon that moves under the same geometrical conditions,
recovering the formulas that account for the relativistic Doppler Effect,
illustrating the behavior of a transformation equation when it should account
for the properties of an electron and for those of a photon as well.
