The work done to raise a mass m from the surface of the earth to a height h, which is equal to the radius of the earth, is

Work done =Change in potential energy                     =Uf-Ui=-GMm(R+h)--GMmRwhere  M is the mass of earth and R  is the radius of earth.∴ W=GMm1R-1(R+h)  Now, h=R ∴W=GMm1R-12R=GMm2R ⇒ W=mgR2                                                 ∵ g=GMR2