25. A 4.8-kg suitcase is sliding across the horizontal floor of an elevator. The coefficient of kinetic friction between the suitcase and the floor is 0.28. a) If the elevator is moving upward at a constant speed of 1.6 m/s, find the kinetic frictional force acting on the suitcase. b) If the elevator is accelerating upward at 1.6 m/s', find the kinetic frictional force acting on the suitcase. c) If the elevator is accelerating downward at 1.6 m/s', find the kinetic frictional force acting on the suitcase