Question

if an object of mass m is taken from the surface of earth (radiusR) to a height 2R. what will be the workdone.

Answer 1

the work done =mgh
=mg2R

Answer 2

gravitational potential energy of a body of mass 'm' on the surface of the earth  radius 'R' and mass 'M' will be

U = -GMm/R

and at a height equal to earth's radius

U' = -GMm/(R+R) = -GMm/(2R)

so, change in potential energy will be

dU = U - U'

or

dU = -GMm/R - [-GMm/(2R)]

= (-GMm/R)[1 - 1/2]

thus, we get

dU = -GMm/2R