Newton demonstrated that the gravitational force on one object caused by another is proportional to the product of the masses of the two objects divided by the square of the distance between them. The gravitational field of an object would then be the force divided by the mass of the object on which the force is exerted. The symbol used for the gravitational field is g, and G is the so-called universal gravitational constant. It is called universal because it is the same for objects made of any material and of any mass—from a 1-kg apple to a galaxy.

The equation that describes the gravitational field at a specific location is g = GM/r² where M is the mass of the attracting object and r is the distance from the center of this mass to the location in space. The gravitational field is a vector quantity. Its direction is toward the center of the attracting object. We’ll consider its magnitude shortly.