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The gravitational potential 
at distance 
from point mass 
is given by the expression
where 
is the gravitational constant. Explain the significance of the negative sign in this expression.
A spherical planet may be assumed to be an isolated point mass with its mass concentrated at its centre. A small mass 
is moving near to, and normal to, the surface of the planet. The mass moves away from the planet through a short distance 
. State and explain why the change in gravitational potential energy 
of the mass is given by the expression.
where 
is the acceleration of free fall.
