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. (See Table 7.1 for the energy content of gasoline.)

(a) What is the magnitude of the force exerted to keep the car moving at constant speed?

(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?

Data given in Table 7.1

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Big Bang Object/phenomenon Energy released in a supernova Fusion of all the hydrogen in Earth's oceans Annual world energy use Large fusion bomb (9 megaton) 1 kg hydrogen (fusion to helium) 1 kg uranium (nuclear fission) Hiroshima-size fission bomb (10 kiloton) 90,000-metric ton aircraft carrier at 30 knots 1 barrel crude oil 1 ton TNT Energy in joules 1068 1044 1034 4x1020 3.8×10¹6 6.4×10¹4 8.0x10¹3 4.2x1013 1.1×1010 5.9x10⁹ 4.2×10⁹