A boy lifts a stack of six identical books from the floor to a 1.2-m-high shelf in 2.5 s. If lifting the books requires 50.8 W of power, what is the mass of each book? A worker uses a pulley system to lift a 21.7-kg box a distance of 12.4 m above the ground. The worker must exert a force of 97 N and pull 28.5 m of rope. What is the efficiency of the system? A child holds onto a string attached to a toy boat and exerts a force of 8.0 N to pull the boat a distance of 7.2 m along a straight shoreline. If the child holds the string at a 15.0 angle with the horizontal, how much work does she do on the toy boat? Use the graph to answer problems 2 and 3. 45.0 Force (N) 30.0 15.0 0.0 1.5 3.0 Displacement (m) 2. The graph shows the force and displacement of an object that is being pushed. How much work is done to push the object 3.0 m?