1) 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 55.0 km³ (mass = 5.50 10¹³ kg), given that the lake has an average height of 36.0 m above the generators? J

(b)

Compare this with the energy stored in a 9-megaton fusion bomb.

E_lake/E_bomb =

2) Suppose a car travels 108 km at a speed of 40.0 m/s, and uses 2.20 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 1.30 10⁸ J per gallon.)

(a)

What is the force (in N) exerted to keep the car moving at constant speed? (Enter a number.) 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.) gallons