__Gravitational fields__ are due to objects having mass. The field extends to infinity, but becomes gets weaker as the distance from the centre of mass of the object increases. It becomes negligible at long distances. The mass of a spherical object can be modelled as a __point mass__ at its centre. __Gravitational field lines__ are used to map gravitational fields – they show the path that a test mass would take in a gravitational field.

__Gravitational field strength__ is defined as the gravitational force exerted per unit mass on a small object placed at that point within the field. It is given by:

The unit of gravitational field strength is Nkg^{-1}. Gravitational fields are one of several forms of a field giving rise to a force.