<Tr> <Td> "</Td> <Td> Law I: Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed . </Td> <Td>" </Td> </Tr> <P> The ancient Greek philosopher Aristotle had the view that all objects have a natural place in the universe: that heavy objects (such as rocks) wanted to be at rest on the Earth and that light objects like smoke wanted to be at rest in the sky and the stars wanted to remain in the heavens . He thought that a body was in its natural state when it was at rest, and for the body to move in a straight line at a constant speed an external agent was needed continually to propel it, otherwise it would stop moving . Galileo Galilei, however, realised that a force is necessary to change the velocity of a body, i.e., acceleration, but no force is needed to maintain its velocity . In other words, Galileo stated that, in the absence of a force, a moving object will continue moving . (The tendency of objects to resist changes in motion was what Johannes Kepler had called inertia .) This insight was refined by Newton, who made it into his first law, also known as the "law of inertia"--no force means no acceleration, and hence the body will maintain its velocity . As Newton's first law is a restatement of the law of inertia which Galileo had already described, Newton appropriately gave credit to Galileo . </P> <P> The law of inertia apparently occurred to several different natural philosophers and scientists independently, including Thomas Hobbes in his Leviathan . The 17th century philosopher and mathematician René Descartes also formulated the law, although he did not perform any experiments to confirm it . </P> <P> Newton's original Latin reads: </P>

Each action has an equal and opposite reaction