Find out potential at `P` and `Q` due to the two point mass system Find out work done by external agent in bringing unit mass from `P` to `Q` Also find work done by gravitational force.
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Explanation

`V_(P1)` = Potential at `P` due to mass m' at `1 = - (Gm)/(l)`
`V_(P2) = - (Gm)/l`
`:. V_(P) = V_(P1) + V_(P2) = - (2Gm)/(l)`
(ii) `V_(Q1) = - (GM)/(l//2) rArr V_(Q2) = - (Gm)/(l//2)`
`:. V_(Q) = V_(Q1) + V_(Q2) =(Gm)/(l//2) - (Gm)/(l//2) = - (4Gm)/(l)`
Force at point `Q =0`
(ii) work done by external agent `=(V_(Q) - V_(P)) xx 1 = (2GM)/(l)`
(iv) work done by gravitational force `=V_(P) - V_(Q) = (2GM)/(l)` .