Fields of Force

The Coulomb field of force between two billiard balls make them decelerate or accelerate. The mass gain as kinetic energy is transferred to the field from the ball on the left-hand side. Then the field transmits the equivalent kinetic energy to the other ball, whose mass increases. However, from the field's point of view, both balls are symmetrically bouncing on each other. This calculus is based on Lorentz's mass gain: M=a+r according to active and reactive mass. 

Action and reaction between two billiard balls is essentially caused by electrostatic fields of force, which are responsible for the well-known Coulomb force. The repulsion effect is easily explainable by the fact that electrons only (rarely protons vs. electrons) come very close together when two pieces of matter collide.