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Question

A police car horn emits a frequency 250Hz when it is at rest. What frequency does a stationary observer hear if the police car sounds its horn while approaching at a speed of 27 ms1 ? What frequency is heard if the horn sounded as the car is leaving at 27 ms1 ? Velocity of sound =340 ms1?

A
271 Hz, 231 Hz
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B
271 Hz, 252 Hz
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C
271 Hz, 225 Hz
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D
271 Hz, 261 Hz
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Solution

The correct option is A 271 Hz, 231 Hz
f=250 HZ
vs=27 ms1
v=340 ms1
f’ = ?
When the car approaches the observer
f=fvvvs=250×34034027=271.6 Hz
When the car moves away from the observer
f=fvv+vs=250×340(340+27)=231.6 Hz

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