Doppler Effect Police Car Question

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SUMMARY

The discussion centers on calculating the frequency perceived by a suspect running away from a police car emitting a siren at 400 Hz, using the Doppler Effect equation. The correct formula applied is f_observed = f*[v + (-vo)]/[v - (+vs)], where the speed of sound (v) is divided by 35 for the police car and by 55 for the suspect. The final calculated frequency that the suspect hears is 404.531 Hz, confirming the correct application of the Doppler Effect principles.

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A police car emitting a siren wail (400 Hz) is driving towards a fleeing suspect at v/35, where v is the speed of sound. The suspect is running away at v/55. What frequency does the suspect hear?

For this problem, I used the Doppler Effect equation, and got 419.361 as my answer. I used + on the top and - on the bottom. Do I need to switch the two?
 
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Since the suspect is going in the direction that is away from the sound source, you use a negative value for the observer speed. The source is moving in the direction toward the observer, so there is a positive source speed. That means you get (using the equation correctly):
f_observed = f*[v+(-vo)]/[v-(+vs)]

so there ends up being a - on both top and bottom. Did you follow that?
 
Thanks, so the answer comes out to be 404.531 Hz?
 

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