A stationary police car emits a sound of frequency 1215 Hz that bounces off of a

Question

A stationary police car emits a sound of frequency 1215 Hz that bounces off of a car on the highway and returns with a frequency of 1280Hz . The police car is right next to the highway, so the moving car is traveling directly toward or away from it. And what frequency would the police car have received if it had been traveling toward the other car at 25.0 m/s ?

My try at solving the first part:

1215=((340+0)/(340+V(source))(1280)

The Answer i got was 18.19 but it's incorrect. Thjanks for the help




Could you give me a step by step on how to do it?

Explanation / Answer

We use the Doppler eqn fL = fS*(v + vL)/(v + vS) Here v = 344m/s & vL = 0 because the source is actually the moving car and the listener is the police car So fL/fS*(340+vS) = 340 1280/1215*(340+vS) = 340 So vS = 340*(1-1285/1215) = -18.19m/s So the car was moving at 18.19m/s the negative indicates the car was moving toward the police car Now we want fL = fS*(340 +25)/(340-18.19) = 1215*(340 +25)/(340-18.19) = 1378.06Hz

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