Isaac Newton is often identified as the founder of modern physics owing to his far reaching contributions to many fields of classical physics (and mathematics). In his theory of gravity, he used the information provided by Copernicus, Brahe, and Kepler. He thought the universality of physics should make the principles by which physics operates applicable everywhere, so the gravity we know here on the Earth's surface should work in the same manner as the gravity that produces the orbits of the planets.
With Kepler's laws and elliptical orbits as a starting point, Newton was able to show that a force that varied inversely with the square of the distance is consistent with the behavior of gravity. The form of his theory of universal gravitation for the size of the force between two massive objects is:
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Fgravity=Gm1m2/r2,
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| where m1 is one of the masses, m2 is the other mass, and r is the distance between them. |
With this theory, he was able to explain the Moon's orbit around the Earth, the orbits of the planets about the Sun, and the force of gravity at the surface of the Earth.
To see how this theory works in practice, you can work with the simulation below that is based on it:
The gravitational forces between all of the objects above are found by applying Newton's law of gravitation to every possible pair of objects to compute the forces between them, and then adding all of the forces experienced by each of the objects individually, in accord with the rules of vector addition. The resulting force on each object then produces the motion observed, following Newton's laws of motion (mainly F = ma).
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