The results obtained in electrostatics using Gauss’s law can in some cases be used to obtain results in gravitation, and vice versa. This is because both follow the inverse-square law of distance. We use the following three rules: (1) For charge in electrostatics, the corresponding quantity in gravitation is mass. (2) Electric intensity of electrostatics corresponds to gravitational intensity, or acceleration due to gravity, in gravitation.(3) The constant k = (4πε0)-1 of electrostatics corresponds to -G in gravitation.The acceleration due to gravity at the surface of the earth is g, and at a distance r from the centre of the earth is gr/R, where R is the radius of the earth. Inside a uniformly charged sphere of radius R and charge Q, the electric intensity at a distance r (r < R) from the centre has the magnitude .(a) kQ{1 - (r/R)2} (b) kQr/R3 (c) kQR/r3 (d) kQ(R - r)/R3