## Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

(a) We can find the frequency $f'$ heard by the stationary observer. $f' = \frac{f}{(1-\frac{v_{source}}{v_{snd}})}$ $f' = \frac{400~Hz}{(1-\frac{25.0~m/s}{340~m/s}~)}$ $f' = 432~Hz$ The frequency heard is 432 Hz. (b) We can find the frequency detected by the person who is approaching. $f' = (1+\frac{v_{obs}}{v_{snd}})~f$ $f' = (1+\frac{25.0~m/s}{340~m/s})~(400~Hz)$ $f' = 429~Hz$ The friend hears a frequency of 429 Hz.