A hydroelectric power facility converts the gravitational potential energy of water behind a dam to electric energy. (For each answer, enter a number.) (a) What is the gravitational potential energy (in J) relative to the generators of a lake of volume 49.0 km3 (mass = 4.90 x 1013 kg), given that the lake has an average height of 50.0 m above the generators? x J (b) Compare this with the energy stored in a 9-megaton fusion bomb. Elake/Ebomb = 0.6389 J Suppose a car travels 108 km at a speed of 35.0 m/s, and uses 2.10 gallons of gasoline. Only 30% of the gasoline goes into useful work by the force that keeps the car moving at constant speed despite friction. (The energy content of gasoline is 1.30 x 108 J per gallon.) (a) What is the force (in N) exerted to keep the car moving at constant speed? (Enter a number.) 758.33 .y N (b) If the required force is directly proportional to speed, how many gallons will be used to drive 108 km at a speed of 28.0 m/s? (Enter a number.) 1.5 X gallons