A comparison between the relativistic and the Newtonian mechanics is carried out inside a single coordinate system without reference to other coordinate systems moving relatively to ours. This allows a more direct understanding of the differences between the two mechanics. As a starting point, the equivalence between mass and energy is assumed and taking the route suggested by T. Theodorsen, the correct dependence of the mass on the velocity is obtained.
Classical Mechanics; Relativistic Mechanics; mass-energy equilavence
