By reference to the definition of gravitational potential, explain why gravitational potential is a negative quantity.
Two stars A and B have their surfaces separated by a distance of 
, as illustrated in the figure below.
Point P lies on the line joining the centres of the two stars. The distance x of point P from the surface of star A may be varied. The variation with distance x of the gravitational potential 
at point P is shown in the figure below.
A rock of mass 
moves along the line joining the centres of the two stars, from star A towards star B. Use data from the figure above to calculate the change in kinetic energy of the rock when it moves from the point where 
to the point where 
. State whether this change is an increase or a decrease.
. The change is an increase.
