<Tr> <Td> Second law: </Td> <Td> In an inertial reference frame, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F = ma . (It is assumed here that the mass m is constant--see below .) </Td> </Tr> <Tr> <Td> Third law: </Td> <Td> When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body . </Td> </Tr> <P> The three laws of motion were first compiled by Isaac Newton in his Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), first published in 1687 . Newton used them to explain and investigate the motion of many physical objects and systems . For example, in the third volume of the text, Newton showed that these laws of motion, combined with his law of universal gravitation, explained Kepler's laws of planetary motion . </P> <P> A fourth law is often also described in the bibliography, which states that forces add up like vectors, that is, that forces obey the principle of superposition . </P>

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