An isolated spherical planet has a diameter of 
. Its mass of 
may be assumed to be a point mass at the centre of the planet.
Show that the gravitational field strength at the surface of the planet is 
A stone of mass 
is raised from the surface of the planet through a vertical height of
. Use the value of field strength given in (a) to determine the change in gravitational potential energy of the stone. Explain your working.
.
