A locomotive engine passes a stationary observer at a speed of 72 km/h emitting a note of frequency 500 Hz. Calculate the frequency of sound heard by the observer <br>(i) before, and
<br>(ii) after the engine passes the observer. <br>The speed of sound in air is 340 m/s.
The frequency of the note changes due to the Doppler effect according to the formula (stationary receiver): f = (Vsound / (Vsound +Vsource)) f0
72 km/h = 20 m/s (i) before: Vsource is positive f = (340/(340 - 20))*500 = 531.25 Hz (ii) after: Vsource is negative f = (340/(340+20))*500 = 472.22 Hz