Two blocks ( P and Q ) with respectively masses 2 kg and 1.5 kg are joined by a massless thread. These blocks are mounted on a frictionless pully which is fixed on the edge of a cube ( S ), as shown in the figure below. Block P is positioned on the top surface which has no friction and block Q is in contact with side-surface, having coefficient friction . The cube ( S ) moves towards the right with acceleration of , where is gravitational acceleration. During this movement the block P and Q remain stationary. The value of is 。 (take )