A 8.70 kg box is sliding across the horizontal floor of an elevator. The coefficient of kinetic friction between the box and the floor is 0.440. Determine the kinetic frictional force that acts on the box for each of the following cases.
(a) The elevator is stationary.
(b) The elevator is accelerating upward with an acceleration whose magnitude is 2.70 m/s2.
(c) The elevator is accelerating downward with an acceleration whose magnitude is 2.70 m/s2.