Gravity

The law is written as F = G·m₁·m₂/r². Strictly, it applies to point masses; it holds equally for spherically symmetric objects, with r the distance between their centres. About a century after Newton, Cavendish measured the constant G, about 6.67 × 10⁻¹¹ N·m²/kg²; the word "universal" means scientists think it applies to masses of any composition and is the same throughout the Universe. G is incredibly small, showing that gravity is very weak: the attraction between masses as small as our bodies, or even objects the size of skyscrapers, is incredibly small — two 1-kg masses 1 m apart pull on each other with about the weight of a grain of pollen. The forces come in pairs: object 1 pulls on object 2 exactly as hard as object 2 pulls on object 1, in the opposite direction.

Although it is the weakest of the four fundamental forces of nature, gravity keeps the planets in orbit around the Sun and the Moon in orbit around Earth; the Moon's pull draws the seas toward it, causing the ocean tides. Einstein described gravity from a very different perspective, as a manifestation of mass warping space and time. He also discovered that gravity pulls not only on mass but on light too.