The tidal force is c secondary effect of the force Of gravity. It arises because the gravitational field is not constant across o body's diameter. When a body is acted on by the gravity Of another body, the gravitational acceleration con vary significantly between the necr side and the far side. This tends to distort shape of the body without altering its volume, Suppose that the gravitational field is due to one other body. Linearising Newton's law Of gravitation around the centre of the reference body yields an approximate inverse cube Along the axis through the centres Of the two bodies, this takes the form