External Problem 4: The three flat blocks are positioned on the 30° incline as shown, and a force P parallel to the incline is applied to the middle block. The upper block is prevented from moving by a wire, which attaches it to the fixed support. The coefficient of static friction for each of the three pairs of mating surfaces is shown. Determine the maximum value, which P may have before any slipping takes place. P-0.30- P- 30 kg 50 kg H, 0.40- 40 kg 30 M.-0.45