posted by glenn .
Explain the law of universal gravitation
Here is a reasonable place to start:
It probably tells you more than you want to know.
I should warn you that Newton's universal "law" of gravity is not perfectly accurate, and has been replaced by the more accurate, and much more complicated, General Theory of Relativity of Einstein. The Einstein theory has met all experimental tests, and correctly predicted black holes, which have been observed indirectly by their effects.