Explain what is meant by a gravitational field.
A spherical planet has mass 
and radius 
. The planet may be considered to have all
its mass concentrated at its centre. A rocket is launched from the surface of the planet such that the rocket moves radially away from the planet. The rocket engines are stopped when the rocket is at a height
above the surface of the planet, as shown in the figure below.
The mass of the rocket, after its engines have been stopped, is 
. Show that, for the rocket to travel from a height to a height 
above the planet’s surface, the change 
in the magnitude of the gravitational potential energy of the rocket is given by the expression
