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Suppose a car travels 108 km at a speed of 40.0 m/s, and uses 1.80 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? 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? gallons

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