Isaac Newtons law of universal gravitation, published in 1687, was the first clear mathematical description of how bodies such as planets and stars attract each other under their mutual gravitational pull.
Newtons inspiration for the theory came from watching an apple falling from a tree. A falling apple accelerates toward the ground, so Newton reasoned from his laws of motion (see Newtons laws of motion) that there must be a force, which he called gravity, acting on the apple. This force might have a huge range and could also be responsible for the orbit of the Moon around the Earth, if the Moon had just theright speed to remain in orbit despite constantly falling?? toward the Earth.
He went on to show that the gravitational force between two massive objects is directly proportional to the product of their masses and weakens with the square of the distance between them. But troublingly, the theory didnt explain why the force was transmitted across empty space. This problem is resolved in Einsteins general relativity theory (see page 18).
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