Newton's law of universal gravitation (Sir I. Newton):
Two bodies attract to each other with equal & opposite forces; the magnitude of this force is proportional to the product of the two masses & is also proportional to the inverse square of the distance among the centers of mass of the two bodies; mathematically specking,
F = (G m M/r^{2}) e,
where m & M are the masses of the two bodies, r refer to the distance between the two and e refer to a unit vector directed from the test mass to the second.