Hi there OK
The ratio of the momentum to the velocity is the relativistic mass, m.
And the relativistic mass and the relativistic kinetic energy are related by the formula:
Einstein wanted to omit the unnatural second term
on the right-hand side, whose only purpose is
to make the energy at rest zero, and to declare
that the particle has a total energy, which obeys:
which is a sum of the rest energy
and the kinetic energy.