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 $27 m s^{- 1}$ ? What frequency is heard if the horn sounded as the car is leaving at $27 m s^{- 1}$ ? Velocity of sound $= 340 m s^{- 1}$ ?

271 Hz, 231 Hz

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271 Hz, 252 Hz

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271 Hz, 225 Hz

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271 Hz, 261 Hz

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Solution

The correct option is A 271 Hz, 231 Hz
$f = 250 H_{Z}$
$v_{s} = 27 m s^{- 1}$
$v = 340 m s^{- 1}$
f’ = ?
When the car approaches the observer
$f^{'} = \frac{f v}{v - v_{s}} = \frac{250 \times 340}{340 - 27} = 271.6 H z$
When the car moves away from the observer
$f^{'} = \frac{f v}{v + v_{s}} = \frac{250 \times 340}{(340 + 27)} = 231.6 H z$

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Similar questions

Q. 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 m s^{- 1}

? What frequency is heard if the horn sounded as the car is leaving at

27 m s^{- 1}

? Velocity of sound

= 340 m s^{- 1}

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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 ms−1 ? What frequency is heard if the horn sounded as the car is leaving at 27 ms−1 ? Velocity of sound =340 ms−1?

The correct option is A 271 Hz, 231 Hzf=250 HZ vs=27 ms−1 v=340 ms−1 f’ = ? When the car approaches the observer f′=fvv−vs=250×340340−27=271.6 Hz When the car moves away from the observer f′=fvv+vs=250×340(340+27)=231.6 Hz