You buy a dog whistle that operates at 21 kHz, but when you blow into it, your dog does not show the slightest reaction. You decide to see if you can hear the whistle yourself. Two friends help you. One drives her car past you while the other blows the whistle from the car’s window. 

(a) How fast must your friend drive the car past you to allow you to hear the whistle, assuming you can hear a maximum frequency of 19 kHz? Give your answer in meters per second and miles per hour. 

(b) If the whistle instead operates at 24 kHz, what speed would the car then need? Would this experiment be feasible? How could you do the experiment with two cars?