Two blocks of masses M and m are connected to each other by a massless string and spring of force constant k as shown in the figure. The spring passes over a frictionless pulley connected rigidly to the edge of a stationary block A. The coefficient of friction between block M and the plane horizontal surface of A is μ. The block M slides over the horizontal top surface of A and block m slides vertically downwards with the same speed. The mass M is equal to