Suppose a car travels 108 km at a speed of 30.0 m/s, and uses 2.0 gal 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 about 140 MJ/ gal. ) What is the magnitude of the force exerted to keep the car moving at constant speed? 666 N 650 N 778 N 888 N