Two blocks of masses M1=4 kg and M2=6 kg are connected by a string of negligible mass passing over a frictionless pulley as shown in the figure below. The coefficient of friction between the block M1 and the horizontal surface is 0.4. When the system is released the masses M1 and M2 start accelerating. What additional mass m should be placed over M1 so that the masses (M1+m) slide with a uniform speed?