To find the gravitational field at point P due to a disc, an infinite number of infinitely thin rings facing P , each with a radius y , width of d y , and mass of d M may be placed inside one another to form a disc. The mass of any one of the rings d M is the mass of the disc multiplied by the ratio of the area of the ring 2 π π y d y to the total area of the disc π π R 2 . So, d M = M ⋅ ⋅ 2 y d y R 2 . Hence, a small change in the gravitational field, E is: d E = G p d M ( p 2 + y 2 ) 3 / 2