The gravitational field is a measure of the gravitational force present in a region of space due to material objects. A quantitative description of the gravitational field is developed in the following way: take the force felt by a test body and divide it by its own mass. The test body may be moved from place to place to map out the size of the field. This equation is Newton's law of universal gravitation extended to a many-particle (or n-particle) system. Although Newton's law of gravitation is applicable in many instances, it is not applicable when relativity (special or general) must be taken into account.

The gravitational field, represented by the symbol g, is defined as the gravitational force per unit mass. For any object whose mass is constant, Newton's second law of motion states that the net external force F on a body is the...