Chapter 9.5, Problem 51E

Wind chill Each winter the so-called “polar vortex” of deep frigid air plunges into the United States and sometimes brings record cold and dangerous wind chills. If s is the wind speed in miles per hour and $s\ge 5$, then the wind chill (in degrees Fahrenheit) for an air temperature of $\text{0°F}$ can be approximated by the function

$\begin{array}{l}f\left(s\right)\text{= }\frac{\text{289}\text{.173 }-\text{ 58}\text{.5731}s}{s+1}\hfill \\ \hfill \end{array}$

(a) At what rate is the wind chill changing when the wind speed is 20 mph?

(b) Explain the meaning of your answer to part (a).