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© Copyright 2000, Jim Loy
Newton discovered the Law of Gravity. Just what is it? Is it as simple as, "Things tend to fall downward?" Of course people knew that things fall downward, ever since there were people. Newton never mentioned being hit on the head by an apple, but he did say that he saw an apple fall. Maybe the moon was in the sky at the time, maybe not. But when Newton saw the apple fall, and thought about it, his great insight was that he saw that the same thing that caused the apple to fall (gravity) was what held the moon in its orbit.
Along with his laws of motion, Newton's law of gravity led directly to mathematical explanations of Galileo's falling object experiments (See Galileo And The Leaning Tower of Pisa), and Kepler's Laws concerning the motions of the planets. Newton's Law of Universal Gravitation states:
F=Gm1m2/d2
F is the force of gravity, G is a constant (the Gravitational Constant) which can be measured, m1 and m2 are the masses of the two objects (earth and apple or earth and moon, in the above text), and d is the distance between them. You can see that gravity follows the famous inverse square law that many physical phenomena follow.
One consequence of all this is that everything attracts everything else with gravitational forces. The earth attracts the apple and the apple attracts the earth with the same force. The apple is the one that moves noticeably because it is so much lighter (and easier to move) than the earth. The earth and a ten pound weight are attracted to each other with a force of exactly ten pounds. In fact, that is the definition of weight, the force of gravitational attraction. Everything attracts everything, hence the "universal" in the name of Newton's law. And the law holds way out as far as telescopes can see; it is truly universal.
Einstein's General Relativity is the more recent, more accurate, law of gravity. Has Newton's law of gravity been thrown onto the trash heap? Is it wrong? No, it is a very valuable first approximation which is good enough for almost all purposes. And Newton's law is valuable for comparison with and understanding Einstein's version. And Einstein's version is too complicated for most purposes.
Note: the above article uses subscripts and superscripts. Here is the law, without these symbols (with M and m being the two masses):
F=GMm/d^2