Answer to Question #105094 in Classical Mechanics for Jeff

Question

A car has a drag coefficient C_{d} = 0.30 Cd=0.30, a frontal area of A=1.9 m2 and a mass 1.2 tonnes. The density of air is 1.2 kg.m^{-3}
ii)What power (in kilowatts) is required to overcome the drag force at this speed?
P_{drag} = P drag= _____ kW.

A car has a drag coefficient C_{d} = 0.30 Cd=0.30, a frontal area of A=1.9 m2 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 ,
Car physics, part 2.
Using the information from above:
ii) What power (in kilowatts) is required to overcome Fr at this speed? P= _____ kW .

Answer 1

The first part that asks about the power required to overcome the drag force:

$$P_{drag}=\frac{1}{2}AC_d\rho v^3=9761\text{ W}$$

at the speed of 110 km/h (make sure you converted kph to m/s by multiplying the kph speed by 1000 and dividing it by 3600).

The rolling resistive force is

$$F_r=C_rN=C_rmg=0.1411\text{ N}.$$

The power required to overcome that force is simply

$$P=F_rv=4.313\text{ W}.$$