The Motion of Bodies
Everything that has hitherto been said concerning the motion of bodies can be summarized in one sentence: the acceleration produced by a force is independent of the body's state of motion, and depends only upon the magnitude and direction of the force. The statement concerning the unalterability of mass can equally well be expressed as follows: Mass is an additive property. That is to say, if one body is added to another with which it combines physically, or chemically, it still retains its own mass, which is added to that of the other body, so that the mass of the whole is equal to the sum of the masses of the constituents. When a force produces acceleration in a body for a considerable time, it is not difficult to determine the velocity and acceleration, from observations made on the path traversed.