Randall D. Knight — Heat Engines and Refrigerators: Problem 40

At fairly slow speeds, a car’s resistance to motion is primarily due to rolling friction. (Air drag becomes significant only at higher speeds.) Suppose a 2200 kg car with rubber tires is driving at 11 mph = 5.0 m/s. The coefficient of rolling friction is 0.020. a. What propulsion power is needed to keep the car moving at a steady speed? b. The car’s drivetrain efficiency (from the motor to the wheels) is 85% and the thermal efficiency of its engine is 20%. The engine burns gasoline, which has an energy density of 130 MJ/gal. What is the car’s mpg (miles per gallon) at this speed?