Dynamics of a point particle

This chapter attributes an inertial ‘mass–energy’ to particles. It also distinguishes between the action of an external field and of long-range and short-range internal forces, which is useful for establishing the laws of dynamics of an interacting body—that is, the equations determining its world line. The chapter also presents the 4-momentum conservation law for massive particles and light particles in inertial reference frames. It then gives some examples which illustrate the role played by this law in collisions. Finally, the chapter illustrates the conservation law by the Compton experiment, that is, the collision of a light corpuscle with a particle, and the concept of the quantum of action that can be derived from it.