The gravitational field strength g at a point is the force per unit mass on a small mass placed there: g = F/m. The unit is N kg-1, which is the same as m s-2.Combining g = F/m with Newton's law F = GMm/r2 gives g = GM/r2 for a point outside a spherical planet of mass M, where r is the distance from the centre.This is an inverse square law. Doubling the distance from the centre reduces g to a quarter of its value.The field around a planet is radial and g is a vector pointing towards the centre. At the Earth's surface g is about 9.81 N kg-1.