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.