Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (4th Edition)

(a) Since the direction of the force of gravity is opposite from the displacement vector, gravity does negative work on the book. The magnitude of the work is equal to the change in potential energy. $W = -mgh$ $W = -(2.0~kg)(9.80~m/s^2)(1.55~m)$ $W = -30.4~J$ (b) Since the direction of the hand's force is in the same direction as the displacement vector, the hand does positive work on the book. The magnitude of the work is equal to the change in potential energy. $W = mgh$ $W = (2.0~kg)(9.80~m/s^2)(1.55~m)$ $W = 30.4~J$