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The Universal Physics

During the plague, Newton lived at Woolsthorpe, where he hypothised that the same force could be responsible for both the fall of an apple from a tree, and the everlasting fall of the Moon toward the Earth. He applied Kepler's third planetary law of periods and distances to the apple and to the Moon, and found that gravitational force had to decrease in proportion to the inverse square of distance from the centre of the Earth.

Newton was not certain of this conclusion until 1679, where was confronted by Hooke's ideas on dynamics. Subsequently, Newton demonstrated that a body subject to a central force, proportional to the inverse-square distance, had to move in accordance with Kepler's first and second laws. Therefore, such laws were astronomical evidence of the existence of a gravitational force depending on the inverse square distance of the planets from the Sun.

Newton included the law of gravitation in the Mathematical Principles of Natural Philosophy, together with the three principles of dynamics. He proved that such principles, valid in terrestrial regions, when joined to the theory of gravitation, could explain perfectly why planets followed Kepler's laws. As a result, Newton's Mathematical Principles completely obliterated the Aristotelian gap between the terrestrial and celestial worlds.

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