A train moving at a speed of $220\,\, m s^{-1}$ towards a stationary object, emits a sound of frequency $1000\,\, Hz.$ Some of the sound reaching the object gets reflected back to the train as echo. The frequency of the echo as detected by the driver of the train is ...... $Hz$
(Speed of sound in air is $330 \,\, m s^{-1}$)
(Speed of sound in air is $330 \,\, m s^{-1}$)
AIPMT 2012, Medium
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Speed of the train, $v_{T}=220 \mathrm{ms}^{-1}$
Speed of sound in air, $v=330 \mathrm{ms}^{-1}$
The frequency of the echo detected by the driver of the train is
$v^{\prime} =v\left(\frac{v+v_{T}}{v-v_{T}}\right)=1000\left(\frac{330+220}{330-220}\right)$
$=1000 \times \frac{550}{110}=5000\,\mathrm{Hz}$
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