  We considered the problem of the proportionality of inertial and
gravitational masses in classical mechanics. We found that the kinetic energy
of a material mass point m in a circular motion with a constant angular
velocity around another material point M depends only on its gravitational
mass. This fact, together with the known result that the straight line is a
circumference with an infinite radius, allowed us to prove the proportionality
between the inertial and gravitational masses.
