A 2.50 kg block is initially at rest on a horizontal surface. A horizontal force F of magnitude 6.90 N and a vertical force P are then applied to the block (see the figure). The coefficients of friction for the block and surface are s = 0.4 and k = 0.25. Decide the magnitude of the frictional force acting on the block if the magnitude of P is (a) 6.00 N and (b) 10.0 N.