Q .     A pair of parallel horizontal conducting rails of negligible resistance is shorted at one end. Rails are fixed on a table. Distance between the rails is `L` . A conducting massless rod of resistance `R` can slide on the rails without any friction. The rod is tied to a massless string which passes over a pulley fixed to the edge of the table. A mass `'m'` tied to the other end of the string hangs vertically. A constant magnetic field `B` exists perpendicular to the table. If the system is released from rest, find acceleration of the mass at the instant when the velocity of the rod is half the terminal velocity.