How Does the Doppler Effect Explain Frequency Changes in Moving Cars?

AI Thread Summary
The discussion revolves around a homework problem involving the Doppler Effect, where a stationary police car emits a sound frequency of 1200 Hz, which is reflected back at 1250 Hz by a moving car. The key to solving the problem is recognizing that the moving car is approaching the police car, indicated by the increase in frequency. The correct speed of the moving car is calculated to be 7.02 m/s, although initial calculations yielded an incorrect result of 13.76 m/s. Participants emphasize the need to apply the Doppler formula twice to find the observed frequencies for both the moving car and the police car. Ultimately, the correct approach leads to a successful resolution of the problem.
Elias Waranoi
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Homework Statement


A stationary police car emits a sound of frequency 1200 Hz that bounces off a car on the highway and returns with a frequency of 1250 Hz. The police car is right next to the highway, so the moving car is traveling directly toward or away from it. (a) How fast was the moving car going? Was it moving toward or away from the police car?

Homework Equations


ƒL = ƒS(v - vL)/(v - vS)

The Attempt at a Solution


The returning frequence is larger so its wave length has to be smaller which tells us that the car is moving towards the police car.

vS = v - vƒSL = 13.76 m/s
The correct answer is 7.02 m/s.
What am I doing wrong?
 
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Hint: You need to apply the Doppler formula twice.

1) What frequency does the moving car "observe"? (That's the frequency it will reflect.)
2) What frequency does the police car observe?
 
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Elias Waranoi said:

Homework Equations

Where fL is what, exactly? Is that the frequency that the police will hear?

Edit: pipped by The Doc.
 
Doc Al said:
Hint: You need to apply the Doppler formula twice.

1) What frequency does the moving car "observe"? (That's the frequency it will reflect.)
2) What frequency does the police car observe?

Thanks! I got it right now.
 
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