Driving on asphalt roads entails very little rolling resistance, so most of the

Question

Driving on asphalt roads entails very little rolling resistance, so most of the energy of the engine goes to overcoming air resistance. But driving slowly in dry sand is another story. If a 1400 kg car is driven in sand at 5.0 m/s , the coefficient of rolling friction is 0.06. In this case, nearly all of the energy that the car uses to move goes to overcoming rolling friction, so you can ignore air drag in this problem. If the car gets 15 miles per gallon when driving on sand, what is the car's efficiency? One gallon of gasoline contains 1.4×108 J of chemical energy, one mile is 1609 m.

Explanation / Answer

15 miles = 24140 m

1 gallon of gasoline contains 1.4×10 J.

So moving a distance of 24140m requires gasoline containing 1.4×10 J

Therefore moving a distance of 1m requires gasoline containing 1.4×10/24140 = 5800 J

Overcoming rolling resitance for 1m requires (useful) work = force x distance = 1000x1 = 1000J

So 5800J (in the gasoline) provides 1000J (overcoming rolling resistance) of useful work for each metre moved.

Efficiency = useful work/total energy supplied
= 1000/5800
= 0.17 (=17%)

Related Questions

Navigate

• Browse (All)
• Browse D
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.