A particle of mass 50 g is kept on the surface of a uniform sphere of mass 120 kg and radius 50 cm. Find the work to be done  in ×10−10J against the gravitational force between them to take the particle far away from the sphere. Take G=6.67×10−11Nm2kg−2.

Here, M=120kg;m=50g=0.05kg           R=50cm=0.5m and G=6.67×10−11Nm2kg−2Now, initial potential energy of the system of sphere and particle,Ui=−GMmR=−6.67×10−11×120×0.050.5=−8.0×10−10JWhen the particle is far away from the sphere, the potential energy of the system will be zero i.e. Uf =0Therefore, required work done,                  W=Uf−Ui=0−−8.0×10−10=8.0×10−10J