Newton's postulate - universal gravity

The attractive force between two uniform spherical masses is given by the relationship above, where G is the gravitational constant.

The weight of an apple, the centripetal force acting on the Moon, and the elliptical orbits of the planets, are direct consequences of the truth of that one simple equation.

An inverse square law is neat, simple, even obvious in a three dimensional universe, but it may not be exact at all distances. Direct definitive measurements beyond the solar system have not yet been made.

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