In his paper Part 8 on the physical nature of velocity, the author has demonstrated that the fact that the size of a particle system is the area that is repetitively covered by the motions of its basic components, leads in a natural way to the Lorentz-Fitzgerald contraction of a moving particle system, which means that the so-called length ‘contraction’ is a real, physical compression in its direction of motion.This allows the author to demonstrate in this paper that the force that we have to exert on a mass particle system to increase its velocity, is in fact the force that we must exert on it to compress it in its direction of motion.It follows from this that, when a 3-dimensional mass particle system reaches the speed of light, it will have been compressed into a 2-dimensional particle system that moves with an invariable speed and that has variable motion and mass behaviour (such as linear momentum and gravitational interaction) in a plane perpendicular to its invariable speed.This means that the inertial ‘mass’ of a particle system is an emergent property that finds its origin in the 3-dimensional cyclic motion of its massless components, which leads in a self-evident way to Einstein’s mass-energy equivalence, and allows the author to give the physical meaning of the Compton and the de Broglie wavelengths and of Planck’s constant, and will in his next paper lead to a quantum theory on gravitation.