You may have heard the legend that, while napping under a tree, Newton was struck
on the head by a falling apple. This alleged accident supposedly prompted him to
imagine that perhaps all objects in the Universe were attracted to each other in the same way the apple was attracted to the Earth. Newton analyzed astronomical data on the motion of the Moon around the Earth. From that analysis, he made the bold assertion that the force law governing the motion of planets was the same as the force law that attracted a falling apple to the Earth.
In 1687, Newton published his work on the law of gravity in his treatise Mathematical
Principles of Natural Philosophy. Newton’s law of universal gravitation states
that