Doppler Effect Homework: Find Speed of Ambulance

AI Thread Summary
To find the speed of the ambulance, the apparent frequencies of 480 Hz when approaching and 420 Hz when receding are used with the Doppler effect equations. The observer's frequency equations are f' = (vf)/(v - vs) for approaching and f' = (vf)/(v + vs) for receding. The user initially struggled to determine the actual frequency emitted by the ambulance, attempting to use an average value of 450 Hz, which was incorrect. After some algebraic manipulation and guidance from others, the user successfully solved the problem. The discussion emphasizes the importance of correctly applying the Doppler effect formulas to find the speed of a moving source.
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Homework Statement


A stationary observer at a crosswalk hears an ambulance siren with an apparent frequency of
480 Hz when the ambulance is approaching. After the ambulance passes the apparent frequency is only 420 Hz. Find the speed of the ambulance. Assume v = 343 m/s for the speed of sound in air.

Homework Equations


source moving towards observer: f' = (vf)/(v - vs)
source moving away from observer: f' = (vf)/(v + vs)

The Attempt at a Solution


I am having trouble because I am not sure how to figure out what the frequency of the sound given off by the ambulance is. I tried using 450 since it is halfway between, and it gave me a close answer, but not the correct one. Any help would be much appreciated.

Thanks a lot
 
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hi asdf12321asdf! :smile:
asdf12321asdf said:
source moving towards observer: f' = (vf)/(v - vs)
source moving away from observer: f' = (vf)/(v + vs)

The Attempt at a Solution


I am having trouble because I am not sure how to figure out what the frequency of the sound given off by the ambulance is. I tried using 450 since it is halfway between, and it gave me a close answer, but not the correct one.

Quelle surprise! :smile:

c'mon … use some algebra! :wink:​
 
oh ok I figured it out thanks a lot
 

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