You are driving on a straight section of the interstate at speed vobs 36.0 m/s (

Question

You are driving on a straight section of the interstate at speed vobs 36.0 m/s (about 81 mph) when you pass a police car stopped on the side of the road. The police car seeing that you are speeding turns on its siren and starts to pursue you. When the police car is at rest the frequency of the siren you hear is f1-588 Hz, and when it matches its speed with your car the frequency you hear is f2 660 Hz. The speed of sound vsound is unknown and there is no wind. Answer the following questions. STOPPED POLICE obs STOPPED POLICE obs STOPPED POLICE Vobs POLICE Dobs source 9. What is the frequency fsource of the police siren at its source?

Explanation / Answer

when the police car is at rest

Vobs = 36 m/s (listener)

Vp = 0 (source)

Vs = speed of sound

since the listener is moving away from the source at rest

f1 = (fsource)(Vs - Vobs ) /Vs

588 = (Vs - 36 ) (fsource)/Vs eq-1

when the police car moves at same speed as ours, then

fsource = frequency observed = 660 Hz

hence no change in frequency

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