We all probably know of the story where Sir Isaac Newton, during his time at Cambridge, sat under an apple tree and an apple fell from the tree right onto his head which in turn sparked the thought of gravity. Newton, in his theory of universal gravitation, states how two bodies feel for each other a mutual attraction, a force he called gravity. He believed this is the same force that makes the apple fall to the Earth and which makes the Moon revolve around the Earth. In mathematical terms, he said that the force of gravity experienced between two bodies is inversely proportional to the square of the distance and is directly proportional to the product of the bodies' masses. The formula is depicted below, where G is a universal constant: