The force imparted by the earth to a mass that is at rest relative to the earth. Since the earth is rotating, the force observed as gravity is the resultant of the force of gravitation and the centrifugal force arising from this rotation. It is directed normal to sea level and to its geopotential surfaces. The magnitude of the force of gravity at sea level decreases from the poles, where the centrifugal force is zero, to the equator, where the centrifugal force is a maximum but directed opposite to the force of gravitation. This difference is accentuated by the shape of the earth, which is nearly that of an oblate spheroid of revolution slightly depressed at the poles. Also, because of the asymmetric distribution of the mass of the earth, the force of gravity is not directed precisely toward the earth's center. The magnitude of the force of gravity per unit mass (acceleration of gravity) g may be determined at any latitude φ and at any geometric height z (meters) above sea level in the free air from the following empirical formula:

where gφ = 980. 6160 (1 − 0. 0026373 cos 2φ + 0. 0000059 cos2 2φ) is the sea level value of gravity (cm s−2) at latitude φ. This formula as applied near the earth indicates that gravity changes very little with height or latitude, so that for rough calculations a constant value of 980 cm s−2 may be used. Besides these variations in the magnitude of the force of gravity, there are more localized variations controlled by the topography of the earth's surface, and the distribution of mass beneath. The magnitude of the force of gravity is usually called either gravity, acceleration of gravity, or apparent gravity. See virtual gravity, geopotential height, standard gravity.