The Earth and the Moon may be considered to be spheres that are isolated in space with their masses concentrated at their centres. The masses of the Earth and the Moon are 
and 
respectively. The radius of the Earth is 
and the separation of the centres of the Earth and the Moon is 
, as illustrated in the figure below (not drawn to scale).
Explain why there is a point between the Earth and the Moon at which the gravitational field strength is zero.
Determine the distance, in terms of , from the centre of the Earth at which the gravitational field strength is zero.
.
