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State what is indicated by the direction of the gravitational field line at a point in a gravitational field.
A large isolated uniform sphere has mass 
and radius 
. Point P lies on a straight line passing through the centre of the sphere, at a variable displacement 
from the centre, as shown in the figure below.
The graph below shows the variation with of the gravitational field 
at point P due to the sphere for
the values of for which P is inside the sphere.
The magnitude of the gravitational field at the surface of the sphere is 
.
Determine an expression for in terms of and . Identify any other symbols that you use.
Explain why, at the surface of the sphere, always has the opposite sign to .
which is the gravitational constant.
