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 1200 kg car is driven in sand at 5.2 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.
a. What propulsion force is needed to keep the car moving forward at a constant speed?

b. What power is required for propulsion at 5.2 m/s?
If the car gets 15 mpg when driving on sand, what is the car's efficiency? Assume the density of gasoline is 719.7 kg/m^3.
Help please!

Answer 1

A) Agreed. 
b) Value agreed but units should be W (watts). 

c) Here's one method... 

15 miles = 24140 m 

1 gallon of gasoline contains 1.4×10⁸ J. 

So moving a distance of 24140m requires gasoline containing 1.4×10⁸ J 

Therefore moving a distance of 1m requires gasoline containing 1.4×10⁸/24140 = 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%)