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Gravity is a force that causes any two bodies of mass to attract each other.

Isaac Newton formulated the following law: two bodies, of mass M and m at a distance apart of r attract each other with a force of GMm/r2 where G is a univeral constant. Its value is 6.67259 x 10-11 Nm2/kg2.

Newton's rule was successful in predicting planetary motions, so much so that minor deviations in the motion of Uranus led to the discovery of Neptune. However, a small discrepancy remained in the motion of Mercury. This discrepancy was one of the factors that led Albert Einstein to formulate his Theory of Relativity.

Gravity is far weaker than electromagnetic forces or sub-atomic ones. However, it affects all matter and operates over long ranges, so it is the dominant force on astronomical scales.