A train moving at a speed of $220\, ms^{-1}$ 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 $330\, ms^{-1}$)
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Frequency of the echo detected by the driver of the train is
(According to Doppler effect in sound)
$f^{\prime}=\left(\frac{v+u}{v-u}\right) f$
where $f=$ original frequency of source of sound
$f^{\prime}=$ Apparent frequency of source because of the relative motion between source and observer.
$f^{\prime}=\left(\frac{330+220}{330-220}\right) 1000=5000 \mathrm{Hz}$
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