Gravity (or more properly, gravitation) is the attraction which all particles of matter exert on one another.
Newton's model says that every particle in the universe attracts every other particle with a force which is directly proportional to the mass of each particle and which varies inversely as the square of the distance between them:
F = G.m1.m2
___________
r2
Here m1 and m2 are the masses, r is the distance between them and G is a fixed quantity called the Universal Gravitational Constant.
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All this means is that if you double either of the masses, the attractive force will double, but if you double the separation, the attractive force will go down to one quarter (1/22) of the original value. G has to be found by experiment; it is very small ( 0.000 000 000 067 newton metre2 / kilogram2 ), showing that gravitation is a very weak force.
Albert Einstein produced a different model of gravitation. He did away with forces altogether ! Instead, he said that
any body possessing mass would produce a distortion in space and time. Empty space, according to Einstein is 'flat' and in this kind of space, bodies travel in straight lines. Space containing bodies having mass is 'warped' so that the most natural path changes from being straight to being curved. On this view, the Moon is not held in its orbit by gravitational forces; it merely follows a natural path in the curved space created by the presence of the Earth.
Hope this helps
Cheers,
Herman
