![]() Langley, Alfred Gideon, New York, 1896, 688– 689) Google Scholar. ![]() ( Leibniz, Gottfreid Wilhelm, New Essays concerning Human Understanding (trans. ![]() Certainly, while A and B meet, the resistance of the bodies, united with their elasticity, causes them to be compressed because of the percussion, and the compression is equal in each … Then the balls A and B, restoring themselves by the force of their own violent … elasticity, mutually repel each other by turns, and spread out, as it were, in an arc, and, with a force equal on both sides, each is driven back by the other, and so, not by the force of the other, but by its own force, it recedes from that one … From what has been said, it is understood that the action of bodies is never without reaction, and both are equal to each other, and directly contrary.” I understand here, however, passion proper, which arises from percussion … For since the percussion is the same, to whatever at length true motion corresponds, it follows that the result of the percussion is distributed equally between both, and thus both act equally in the encounter, and thus half the result arises from the action of the one, the other half from the action of the other and since half, also, of the result or passion is in one, half in the other, it is sufficient that we derive the passion which is in one from the action also which is in itself, and we need no influence of the one upon the other, although by the action of one an occasion is furnished the other for producing a change in itself. “the passion of every body is spontaneous, or arises from internal force, although upon external occasion. It is interesting to note that in the passage leading up to his own formulation of the action-reaction law (composed in 1695), Leibniz left the meaning of “action” and “reaction” equally vague, even though his discussion was confined to impact phenomena, one field to which Newton had applied his third law in very precise terms. The action is to be assessed directly in terms of the light emitted, the reaction is to be assessed in terms of the heat generated. The first passage just quoted shows, however, that the reduction to such forces is not necessary in order to assess the action and reaction. 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.ĩ It is perfectly true, of course, that in terms of Newton's particulate theory of light, the action and reaction between a body and light may be reduced ultimately to accelerative forces between particles of the body and particles of light. 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. ![]() 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. 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.
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