2 When an automobile moves with a constant speed down a highway, most of
3 the power developed in the engine is used to compensate for the energy
4 transformations due to friction forces exerted on the car by the air
5 and the road. If the power developed by the engine is $175\U{hp}$,
6 estimate the total friction force acting on the car when it is moving
7 at a speed of $29\U{m/s}$. One horsepower equals $746\U{W}$.
11 The energy going into friction in a distance $x$ is
13 E_\text{int} = -W_f = -F_f x\cos(180\dg) = F_f x \;.
16 The power going into friction is thus
18 P_f = \deriv{t}{E_\text{int}} = F_f \deriv{t}{x} = F_f v \;.
21 If the power lost to friction matches the power generated by the
22 engine, then the force of friction is given by
26 = \frac{175\U{hp}\cdot\frac{746\U{W}}{1\U{hp}}}{29\U{m/s}}
27 = \ans{4.50\U{kN}} \;.