1. The problem statement, all variables and given/known data A factory whistle emits sound of frequency f. On a day when the wind velocity is w from the north, what frequency will observers hear who are located, at rest, (a) due north, (b) due south, (c) due east, and (d) due west, of the whistle? What frequency is heard by a cyclist heading (e) north or (f) west, toward the whistle at a speed u? 2. Relevant equations Frequency heard by stationary observer of a sound source (1) moving toward the observer: f/(1 - vs/v); (2) moving away from the observer: f/(1 + vs/v). Frequency heard by a moving observer of a stationary sound source when (3) the observer is moving toward the source: f(1 + vo/v); (4) the observer is moving away from the source: f(1 + vo/v). f is the frequency of the sound source, v is the velocity of sound in air, vo is the velocity of the observer and vs is the velocity of the sound source. 3. The attempt at a solution This problem is a bit tricky for me because it discusses a new concept: waves on a moving medium, viz. sound waves in the wind. I thought of a simpler situation: a transverse wave on a moving string. In this situation, if the wave moves in the same direction as the string, an observer will see a faster moving wave than one in which the string is not moving right? The question is: Do I consider the wind as a moving sound source then?