posted by Set11 on .
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.
a) F = M*g*0.06 = 705.6 N
b) Power = F*V
c) For efficiency, you need the heating value of the fuel as well as the mpg.
Fuel required to go one mile = 1/15 gallon = 0.257 liters = 2.57*10^-4 m^3
Mass of fuel needed per mile = 0.185 kg
Multiply that by the heat value for the chemical energy input.
The delivered power out per mile is F*1609 meters = 1.14*10^6 J
Efficiency = (mechanical work out)/(chemical energy in)