What do we mean by dynamics? It is the branch of mechanics that is concerned with the forces that change or produce the motion of particles or bodies. It is distinct from kinematics which is concerned with the mechanics of particles or bodies without reference to the forces involved. Thus Chapter 3 on relativistic kinematics investigated conservation of energy and momentum in particle collisions and decays without reference to the forces and interactions causing these processes. Electrodynamics obviously defines the dynamics when the forces are electromagnetic in origin.