The Discovery of Universal Gravitation

The first major theory of gravity came in 1666. Before this date, scientists assumed that the force that keeps people, houses, trees, and mountains firmly in place on Earth and the force that keeps the Earth in orbit around the Sun were separate and distinct attractions. Then a brilliant young Englishman named Isaac Newton showed that this was not the case; at the same time, he demonstrated how gravity actually works.

According to Newton, he got his first major clue to gravity’s identity when he witnessed an apple falling from a tree. He was not surprised that the apple fell and struck the ground, of course, since it had long been common knowledge

English scientist Isaac Newton introduced a mathematical formula to measure the gravitational pull of objects.

that some mysterious power drew all objects toward the center of the Earth. What suddenly piqued Newton’s interest was the concept of distance as it related to the mystery force. It occurred to him that if he stood at the top of the tallest mountain in the world and tossed out an apple, the apple would fall to the ground just as surely as it did from the branch of the tree. This meant that the mystery force was strong enough to pull on objects over distances of tens of thousands of feet. Perhaps, then, that force might pull on objects lying much farther away.

This naturally led Newton to think about the Moon, which was clearly hundreds of thousands of miles away from Earth. Maybe, he reasoned, the same force that caused the apple to fall was pulling on the Moon. In that case, the Moon was “falling” toward Earth and the only reason the two objects did not collide was that the Moon’s rapid motion outward, into space, cancelled out, or balanced, the attraction of the mystery force. From this logical (and as it turned out, correct) realization, it was not a great leap to suppose that the very same force kept Earth and the other planets in orbit around the Sun. Newton concluded that the mystery force, which he called gravity, existed throughout the universe, and was therefore universal. And appropriately, he dubbed his new theory the law of universal gravitation.

Through an elegant mathematical formula, Newton demonstrated that the gravitational pull exerted between two objects depends on two factors the mass of the objects and the distance separating them. A small object with very little mass, he showed, exerts very little attraction on another object; a very large and massive object, such as a planet, exerts a measurable gravitational pull on another object. At the same time, distance comes into play. The farther apart two objects are, Newton showed, the less their gravities attract each other. And the reverse is also true the closer the two objects are, the stronger they attract each other. This explains why the Sun easily maintains its hold on Earth, which lies relatively near the star, while the Sun’s gravity has no measurable effect on other stars, which exist at distances thousands of times greater than that between the Sun and Earth.

Newton’s theory of universal gravitation revolutionized the physical sciences, especially the disciplines of physics and astronomy. As noted science writer John Gribbin states it:

Newton really had explained the fall of an apple and the motion of the Moon with one set of laws. In doing so, he removed the mystery from the behavior of heavenly bodies, and opened the eyes of scientists to the fact that the behavior of the stars and planets the behavior of the whole universe might be explained using the same laws of physics that are derived from studies carried out in laboratories on Earth.