The gravitational potential gradient at a point is the change in gravitational potential per metre, ΔV/Δr. Its unit is J kg-1 m-1, which is the same as N kg-1.The gravitational field strength is equal to the negative of the potential gradient: g = -ΔV/Δr. The minus sign shows that the field points in the direction of decreasing potential.On a graph of V against r, g at any point is found from the gradient of the tangent to the curve.Equipotentials are surfaces of equal potential. They are perpendicular to the field lines, and no work is done moving a mass along an equipotential.