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 1700 kgcar is driven in sand at 5.1 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.

What propulsion force is needed to keep the car moving forward at a constant speed?

Express your answer to two significant figures and include the appropriate units.

What power is required for propulsion at 5.1 m/s ?

Express your answer to two significant figures and include the appropriate units.

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.

Express your answer using two significant figures.

Explanation / Answer

a)
Force needed to keep the car moving is equal and opposite to the frictional force that is acting

F = miu*m*g
=0.06*1700*9.8
=999.6 N
Since you want answer in 2 significant figure
Answer: 1.0*10^3 N

b)
Power= F * V
= 999.6*5.1
= 5.1*10^3 W
Answer: 5.1*10^3 W

c)
15 miles = 15*1609 = 24135 m
1 gallon = 1.4*10^8 J

If speed is 5.1 m/s, distance travelled in 1 s = 5.1 m

Energy supplied by fuel to go 5.1 m = 1.4*10^8 * 5.1 / 24135
= 3.0*10^4 J

But in 1 second, energy required = 5.1*10^3 J

effeciency = useful energy * 100 / total energy
=(5.1*10^3)*100 / (3.0*10^4)
= 17 %
Answer: 17 %

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