Question
Derive an expression for the potential energy stored in the mass m placed in the gravitational field of mass M.
Solution
Gravitational potential energy stored in the mass m is the work done against the gravitational force of mass M to bring it from infinity to that point without acceleration.
Let the mass m be at P, at a distance r from the mass M.
The gravitational force on m exerted by M is,
If m is moved by the distance 'dr' towards M, then work done by gravitational force is,
Total work done when the mass m is moved from infinity to a distance r is,
This work by definition is equal to gravitational potential energy,
∴
where, V is the gravitational potential due to mass M at a distance r.

Let the mass m be at P, at a distance r from the mass M.
The gravitational force on m exerted by M is,

If m is moved by the distance 'dr' towards M, then work done by gravitational force is,
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Total work done when the mass m is moved from infinity to a distance r is,
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This work by definition is equal to gravitational potential energy,
∴
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where, V is the gravitational potential due to mass M at a distance r.