blocks and pulley

Problem 511
Find the least value of P required to cause the system of blocks shown in Fig. P-511 to have impending motion to the left. The coefficient of friction under each block is 0.20.
 

Problem 510
What weight W is necessary to start the system of blocks shown in Fig. P-510 moving to the right? The coefficient of friction is 0.10 and the pulleys are assumed to be frictionless.
 

Problem 509
The blocks shown in Fig. P-509 are connected by flexible, inextensible cords passing over frictionless pulleys. At A the coefficients of friction are μ_s = 0.30 and μ_k = 0.20 while at B they are μ_s = 0.40 and μ_k = 0.30. Compute the magnitude and direction of the friction force acting on each block.