 Following Griffiths’ approach, we might say that so far we have merely defined some quantities (Griffiths 1999, p. 509 ff.). The physics really lies in the three corresponding conservation principles associated with these quantities: the principle of conservation of mass, the principle of conservation of energy, the principle of conservation of (linear) momentum. These principles contain the physics, because each one states that a certain quantity, mass, energy, or momentum, is conserved in all interactions. So, for example, if we consider the collision of two billiard balls, one can use the conservation of momentum to predict, given the initial motions and masses of the billiard balls, how they will move after the collision. 