physics

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A siren mounted on the roof of a firehouse emits sound at a frequency of 900 Hz. A steady wind is blowing with a speed of 15 m/s. Take the speed of sound in calm air to be 343 m/s.

(c) Firefighters are approaching the siren from various directions at 15.0
m/s. What frequency does a firefighter hear if he or she is approaching
from an upwind position, so that he or she is moving in the direction
in which the wind is blowing?


_____Hz



(d) What frequency does a firefighter hear if he or she is approaching from a downward position and moving against the wind?


I know that for part C the Observer is traveling at 0m/s with respect to the wind and for Part D the observer is moving at 30m/s with respect to the wind. I know that the speed for the source is 15m/s.

I need help figuring out why the source is traveling towards observer for part C and away for part C. I don't get the direction of the sound from the source. Basically I am having trouble with setting up the correct denominators.

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