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In Aristotle’ physics, objects simply returned to Earth because everything was thought to return to its natural state, and the natural state of objects was to be on the ground. Newton improved on Aristotle’s explanation by providing a mathematical expression for the gravitational force between two objects and by identifying the force which causes apples to fall as the same force that keeps planets in their orbits. The latter is the more subtle and profound of Newton’s contributions because until Newton, people believed that planets were kept in place by divine purpose. Newton’s laws of universal gravitation provided a physical explanation for the patterns in planetary motion.

But Newton’s law of universal gravitation extends gravity beyond earth. Newton’s law of universal gravitation is about the universality of gravity. Newton’s place in the Gravity Hall of Fame is not due to his discovery of gravity, but rather due to his discovery that gravitation is universal. ALL objects attract each other with a force of gravitational attraction. So as you sit in your seat in the physics classroom, you are gravitationally attracted to your lab partner, to the desk you are working at, and even to your physics book.

Newton knew that the force which caused the apple’s acceleration (gravity) must be dependent upon the mass of the apple. And since the force acting to cause the apple’s downward acceleration also causes the earth’s upward acceleration (Newton’s third law), that force must also depend upon the mass of the earth. So for Newton, the force of gravity acting between the earth and any other object is directly proportional to the mass of the earth, directly proportional to the mass of the object, and inversely proportional to the square of the distance which separates the centers of the earth and the object.

Today, Newton’s law of universal gravitation is a widely accepted theory. It guides the efforts of scientists in their study of planetary orbits.

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