4.37 A force F acts on a 10kg block sliding on a flat surface inclined 30 from the horizontal, as shown in Figure 4.48. Suppose that F=63N. Then the block slides at a constant speed up the inclined surface. (a) What frictional force acts on the block? (b) What is k for the block and surface? (c) If the block slides down the surface with F=0, what frictional force acts on the block? What is its acceleration