A smooth rod OP is fixed vertically. A disc of mass m and radius R is rolling without slipping at the given instant on a rough horizontal surface as shown. Velocity of centre of mass C of disc is v 0 . A rod AB of length L is connected with disc at A, R/2 distance vertically up from centre of disc with pin joint (about which it can rotate freely), other end of the rod is attached to a small smooth ring B, which can move freely over rod OP. At an instant rod AB makes an angle 60° with the vertical. The velocity of ring on the vertical rod at this instant is.