ap = 0 and vB = 0 ⇒ V - rω = 0 and α - rα = 0 ⇒ v = rω and a = rα Since, the body acceleration 'a' is downwards parallel to the plane. Therefore, acceleration of the body must be in anti-clockwise sense. That means the friction must act up the plane to produce an anti-clockwise torque to produce anti-clockwise angular acceleration.