"If a number of equal forces act in the same direction, each on a unit of mass, the different masses will all move in the same manner, and may be joined together into one body without altering the phenomenon. The velocity of the whole body is equal to that produced by one of the forces acting on a unit of mass. Hence the force required to produce a given change of velocity in a given time is proportional to the number of units of mass of which the body consists."