Two ends A and B of a smooth chain of mass `m` and length `l` are situated as shown in figure. If an external agent pulls A till it comes to same level of B, work done by external agent is

Options

A. (a) `(mgl)/(36)`
B. (b) `(mgl)/(15)`
C. (c) `(mgl)/(9)`
D. (d) None of the above

Answer

A

Explanation

Final figure is shown below. It can be obtained as follows.
We can assume that a part of length `l//6` is cut from the lowe portion of side B, and put below A.
Mass of this part `=m/6`
This part rises by `=l/6`
Work done=Increase in PE`=(m)/(6l)gl/6=(mgl)/(36)`