A particle of mass m is placed at the centre of a uniform spherical shell of mass 3m and radius R. The gravitational potential on the surface of the shell is

Gravitational potential on the surface of the shell is V = Gravitational potential due to particle (${\mathrm{V}}_1$) + Gravitational potential due to shell itself (${\mathrm{V}}_2$)

   = $-\frac{\mathrm{Gm}}{\mathrm{R}}+(-\frac{\mathrm{G}3\mathrm{m}}{\mathrm{R}}) = -\frac{4\mathrm{Gm}}{\mathrm{R}}$