A smooth narrow tube in the form of an arc AB of a circle of center O and radius r is fixed so that A is vertically above O and OB is horizontal. Particles P and Q of mass m and 2m respectively. with a light in extensible string connecting them (length $\large\frac{\pi\;r}{2}$) are placed inside the tube with P at A and Q at B. and released from rest. Assuming that the string remains during motion, find speed of particles when P reaches B

Question

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