  In a previous paper \cite{MTAY1} we have shown that Newton'n third law cannot
strictly hold in a distributed system of which the different parts are at a
finite distance from each other. This is due to the finite speed of signal
propagation which cannot exceed the speed of light at vacuum, which in turn
means that when summing the total force in the system the force does not add up
to zero. This was demonstrated in a specific example of two current loops with
time dependent currents, the above analysis led to suggestion of a relativistic
engine \cite{MTAY3,AY1}. Since the system is effected by a total force for a
finite period of time this means that the system acquires mechanical momentum
and energy, the question then arises if we need to abandon the law of momentum
and energy conservation. The subject of momentum conversation was discussed in
\cite{MTAY4}. Here some preliminary aspects of the exchange of energy between
the mechanical part of the relativistic engine and the electromagnetic field
are discussed. We also refer briefly to the material composition, structure and
properties of metals that should be used in a relativistic engine.
