  The expressions of momentum and energy of a particle in special relativity
are often derived in a quite unconvincing manner in elementary text, by
resorting either to electrodynamic or quantum considerations, or via the
introduction of the less-than-elementary concept of a four-vector. It is
instead possible, by exploiting considerations introduced by P. Epstein and A.
Einstein and exploited later by Feynman, to obtain a fully elementary
derivation of these expressions and of the $E=mc^2$ formula exploiting only
Lorentz transformations and the postulate of the conservation of quantities
defined for point-like particles which reduce to the Newtonian expressions of
momentum and energy in the classical limit.
