The Third Law in Newton's Mechanics

Abstract

Most modern analysts of Newton's laws of motion, whether they have approached the subject from a historical or from a philosophical viewpoint, have tended to concentrate on the status of the first two laws; the third law has largely been overlooked, or else it has been dismissed as somehow less interesting. My purpose in this paper is to reverse this approach—I intend to investigate some of the historical aspects of the third law, particularly the empirical background to Newton's statement of it, and in so doing, I intend to skirt most of the questions which have been raised concerning the status of the other two laws. In concentrating on the historical aspects of the third law, I shall also by-pass Mach's controversial re-interpretation of its role in mechanics, for while Mach saw the law as the basis for an operational definition of “mass”, it is quite clear that Newton did not so regard it. On the contrary, Newton seems to have regarded all three of his laws as straightforward statements of fact about the world, so that a knowledge of the factual background to the laws is a fundamental pre-requisite to an understanding of Newton's thought.