QUANTITY OF MOTION: MOMENTUM
There is more to defining the motion of a body than simply finding a value for its velocity. For example, in the collision of two billiard balls moving with different velocities, it may appear that they simply exchange their states of motion so that the total quantity of motion is conserved and just redistributed between them. But in the case of two billiard balls of different masses moving with the same speed, the more massive one is able to transfer a greater qu'antity of motion in a collision than the small-mass ball can. Thus the concept of quantity of motion, or momentum, involves both the velocity and the mass of the body. To find momentum, we multiply the body's mass by its velocity.
It is not difficult to visualize philosophically the possibility that the total quantity of motion, or momentum, in the universe is a constant-that is, the total momentum of the universe is conserved although the interaction of various bodies in the universe with each other redistributes the momentum of individual bodies. This concept of conservation of momentum was first annunciated by the French natural philosopher Rene Descartes in his book, Principles of Philosophy, published in 1644.
