Answer to Question #105115 in Classical Mechanics for Henderson

Question

1. A car has a drag coefficient Cd = 0.30, a frontal area of A = 1.9 m^2 and a mass 1.2 tonnes. It has a coefficient of rolling resistance,Cr​, = 0.012. The rolling resistance is a force Fr​ = CrN, where N is the normal force.

i) What is Fr​ when traveling at v = 110 kph on a horizontal road? F= _____ N ,

Hint: Retain accurate values until the final calculation, but remember significant figures.

ii) Using the information from above:
What power (in kilowatts) is required to overcome Fr​ at this speed? P= _____ kW .

Hint: Retain accurate values until the final calculation, but remember significant figures.

Answer 1

As per the given question,

Drag coefficient $C_d=0.3$

Frontal area $= 1.9m^2$

Mass =1.2 tonnes

Coefficient of rolling resistance Cr=0.012

Rolling resistance $F_r​ = C_rN$

Where N = Normal reaction.

i) Speed $= 110 Kph=\dfrac{110\times 1000}{3600}=30.56m /sec$

Friction force does not depend on the velocity,

So $F_r= C_rN=C_r mg=0.012\times1200\times9.8 =141.12 N$

ii) Power P=$F_r v=141.12\times30.56=4312.67J/sec \simeq 4.31 Kwatt$