Two blocks A and B of masses m = 10 kg and M = 20 kg respectively are placed on each other and their combination rests on a fixed horizontal surface C. A light string passing over the smooth light pulley is used to contact A and B as shown. The coefficient of sliding friction between all surfaces in contact is μ = 14. If A is dragged with a force F then for both A and B to move with a uniform speed , the value of F should be