Newton’s Law of Universal Gravitation Every object in the Universe attracts every other object with a force directed along the line of centers for the two objects that is proportional to the product of their masses and inversely proportional to the square of the separation between the two objects. It was Newton who reasoned that the same force that causes an apple to fall from a tree causes the Moon to “fall” in its circular orbit. It was Newton who reasoned that the force that exists between all masses in the universe is directly proportional to the product of their masses. It was Newton who reasoned that the force of attraction between two masses got smaller as the distance between the masses gets larger – to be more specific, the force of gravity is inversely proportional to the square of the distance between the objects. It was Newton who reasoned that masses attract other masses from their center of mass (this is why we feel a force of gravity when standing on the Earth’s surface – the center of mass of the Earth is its center. Newton was aware of the gravitational constant G, but its exact value was determined later as our ability to measure motion improved.