(a) Determine the work a hiker must do on a 15.0-kg backpack to carry it up a hill of height = 10.0 m, as shown in Fig. 7-4a. Determine also (b) the work done by gravity on the backpack, and (c) the net work done on the backpack. For simplicity, assume the motion is smooth and at constant velocity (i.e., acceleration is zero).