\[(a)\;\sqrt {(1+\pi) gr} \quad (b)\; \sqrt {\frac{2}{3}(1+\pi) gr} \quad (c)\;\sqrt {2 \pi gr} \quad (d)\; \sqrt {\frac{1}{3} \pi g r}\]

All surfaces are smooth so, mechanical energy of the system will be conserved.

Decrease in P.E of both the block= Increase in KE of both the Blocks

When P reaches B it has lost height=r

and B will loose height $=\large\frac{\pi r}{2}$, length of string

$\therefore mgr+2mg \bigg(\large\frac{\pi r}{2}\bigg)=\frac{1}{2} $$(m+2m)v^2$

$v= \sqrt {\large\frac{2}{3} \normalsize (1+\pi) gr}$

Hence b is the correct answer

Can u explain to me why B will lose height pie*r/2..i cant understand,plz help!

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