Each particle produces a gravitational field around it which exerts the gravitational force on each of the other particles present within this region. The portion around a point mass in which each other point mass experience a gravitational force, is known as the gravitational field i.e. The intensity of gravitational field at point is the gravitational force exerted on a unit mass reserved at that point. 

The force performing on a particle of mass m placed at a distance r from the centre of earth is given by