A 2.20 kg block is initially at rest on a horizontal surface. A horizontal force of magnitude 4.92 N and a vertical force 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. Determine the magnitude of the frictional force acting on the block if the magnitude of is (a)8.00 N and (b)12.0 N. (The upward pull is insufficient to move the block vertically.)