A spherical planet has diameter 
. The gravitational field strength at the surface of the planet is 
. The planet may be assumed to be isolated in space and to have its mass concentrated at its centre.
Calculate the mass of the planet.
The gravitational potential at a point X above the surface of the planet in (a) is 
. For point Y above the surface of the planet, the gravitational potential is 
. State, with a reason, whether point X or point Y is nearer to the planet.
.
