![]() ![]() So, the third law is what gives the first two laws any predictive power. ![]() Finally, using Newton's third law you can predict that ball B should also be accelerating towards ball A. Using Newton's second law you know that the force is along a vector connecting the two balls. Using Newton's first law you know there is a force acting on ball A from ball B. You see ball A accelerate towards ball B. Kleppner is referring to when he says that Newton's third law is "an important logical element in making sense of the first two laws."įor example, suppose you are watching two balls float in outer space ball A and ball B. Taken by themselves (by which I mean without any reference to any explicit form for the fundamental forces acting between particles), the third law is what gives the first two laws any predictive power. This restriction gives meaning to the "force" described in the first two laws. ![]() ![]() Newton's third law of motion gives meaning to the first two laws by restricting what type of fundamental forces act between particles. Kreuzer, "Experimental measurement of the equivalence of active and passive gravitational mass," Phys. If they were unknown, then Kleppner's proposed test doesn't work: we can't check whether they're accelerating due to the third-law partner of the force they exerted on our experiment.īartlett and van Buren, Phys. But these distant bodies would have to be unknown, or else we could account for their effects. In principle, we could worry that a non-null result from an experiment such as Battat's could be interfered with by gravitational forces from distant bodies - gravity is, after all, a long-range force. The three experiments I've described all happen to be gravitational experiments. We need an independent way of telling whether or not there is a physical interaction on a system. What prevents us from explaining away the difficulty by attributing the acceleration to carelessness in isolating the system? If this option is open to us, Newton's second law becomes meaningless. Suppose that an isolated body starts to accelerate in defiance of Newton's second law. Battat 2007 is a test of the first law that is independent of the third. For examples of experiments that directly test the third law, see Bartlett 1986 and Kreuzer 1968. In my opinion he is still overstating the connection. He's simply saying that there's a connection, not that they are conjoined twins. Kleppner's statement is carefully worded so that he is not claiming (1) that the third law follows from the first and second, nor (2) that the third law is necessary in order to make sense of the first and second laws. Tool, but it is also an important logical element in making sense of Thus Newton's third law is not only a vitally important dynamical If we find such aįorce, the dilemma is resolved the body was not completely isolated. If the acceleration of a body is the result ofĪn outside force, then somewhere in the universe there must be anĮqual and opposite force acting on another body. Or not there is a physical interaction on a system. We need an independent way of telling whether Isolating the system? If this option is open to us, Newton's second What prevents us from explaining away theĭifficulty by attributing the acceleration to carelessness in Suppose that an isolated body starts to accelerate in defiance of I'll post the excerpt if anyone would like to see it.)Įxcerpt from Introduction to Mechanics by Kleppner & Kolenkow (1973), p. (In Kleppner's Mechanics the author states that the third law is a necessity to make sense of the second law. But it seems to me that the third law is not related to the first two, at least logically. The first law and second laws of motion are obviously connected. ![]()
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