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 $$\mu$$. The cube (S) moves towards the right with acceleration of $$\frac{g}{2}$$, where g is gravitational acceleration. During this movement the block P and Q remain stationary. The value of $$\mu$$ is _______.
(take g = 10 m/s$$^2$$)