# Gravitational force Image of the Earth-Moon system taken by Galileo in 1992. Source: NASA.

After stating the laws of the motion of bodies, Newton wondered what was the force responsible for the elliptical orbits described by the planets around the Sun. In his book “Philosophiæ Naturalis Principia Mathematica“, published in 1687, he formulates the idea that the force that keeps the planets in their orbit must be inversely proportional to the square of the distance that separates them from the Sun, but also that this same force is responsible for the orbit of the Moon around the Earth.

The gravitational force exerted mutually by two bodies of mass m1 and m2 is given by Newton’s law of universal gravitation: where r is the distance between the two masses (when they are two large bodies far apart from each other, between their centers), and G a constant of proportionality which, in the units of the International System, has the value:

G = 6.673×10−11 N m2 / kg2

G is called the universal gravitational constant and was measured by Cavendish in 1798, 71 years after Newton’s death. Knowledge is free, but servers are not. Please consider supporting us by disabling your ad blocker on YouPhysics. Thanks!

The constant G is universal because it has the same value everywhere: the gravitational force between two given masses located at the same distance will have the same value regardless of their location.