Gravitational Field Strength - g.

The gravitational field strength g at a point is the gravitational force per unit mass acting on a small test mass placed there, g = F/m. It is measured in N kg^-1 and is a vector that points towards the mass creating the field.Combining g = F/m with Newton's law of gravitation, F = GMm/r², gives g = GM/r² for a point mass M. A spherical planet behaves as if all its mass were concentrated at its centre, so the same equation applies at any distance r from the centre that is at least the planet's radius R.Outside the planet, g obeys an inverse square law: doubling the distance from the centre reduces g to one quarter. At the surface g = GM/R², which is about 9.81 N kg^-1 for the Earth.Close to the surface, over heights that are small compared with R, the field lines are almost parallel and equally spaced, so the field can be treated as uniform. Further away the field is radial and weakens with distance.