Acceleration due to gravity

An object with mass m near the surface of the Earth experiences a downward gravitational force of magnitude mg, where g is the acceleration due to gravity. The quantity g has the dimension of acceleration, m s&minus;2, hence its name.

Newton's gravitational law gives the following formula for g,
 * $$g = G\, \frac{M_{\mathrm{E}}}{R^2_{\mathrm{E}}},$$

where G is the universal gravitational constant, G = 6.67384 &times; 10&minus;11 m3 kg&minus;1 s&minus;2, ME is the total mass of the Earth, and RE is the radius of the Earth. This equation gives a good approximation, but is not exact. Deviations are caused by the centrifugal force due to the rotation of the Earth around its axis, non-sphericity of the Earth, and the non-homogeneity of the composition of the Earth. These effects cause g to vary roughly &plusmn; 0.01 around the value 9.8 m s&minus;2 from place to place on the surface of the Earth. The quantity g is therefore referred to as the local gravitational acceleration.

The 3rd General Conference on Weights and Measures (Conférence Générale des Poids et Mesures, CGPM) defined in 1901 a standard value denoted as gn. The value of the standard acceleration due to gravity gn is 9.80665 m s&minus;2. This value of gn was the conventional reference for calculating the now obsolete unit of force, the kilogram force.