Two links OA and OB are connected by a pin joint at ‘O’. the link OA turns with angular velocity $\omega _{1}$ radians per second in the clockwise direction and the link OB turns with angular velocity $\omega _{2}$ radians per second in the anticloclwise direction. If the radius of the pin at ‘O’. is ‘r’, then the rubbing velocity at the pin joint ‘O’ will be
