Law of universal gravitation

All bodies attract each other. The law of universal gravitation: the force of attraction between two material points is directly proportional to the product of their masses and inversely proportional to the square of the distance between them: F = G·m₁m₂/r². Here G is the gravitational constant, G = 6.67 · 10⁻¹¹ N·m²/kg², first measured by Cavendish. As a pair of forces, each body attracts the other with the same force. The law is exact for material points and for spherical bodies with r the distance between their centres. Because G is tiny, the attraction between everyday objects is unnoticeable; only bodies of planetary mass attract noticeably. If the distance doubles, the force falls 4 times.