Two point masses are isolated in space and are separated by a distance 
. State an expression relating the gravitational force 
between the two masses to the magnitudes 
and 
of the masses. State the name of any other symbol used.
A spacecraft is to be put into a circular orbit about a spherical planet. The planet may be considered to be isolated in space. The mass of the planet, assumed to be concentrated at its centre, is 
. The radius of the planet is 
. The spacecraft is to orbit the planet at a height of 
above the surface of the planet. At this altitude, there is no atmosphere. Show that the speed of the spacecraft in its orbit is 
.
