The driver of a car travelling with speed $30 \,\,m/s$ towards a hill sounds a horn of frequency $600\,\, Hz.$ If the velocity of sound in air is $330 \,\,m/s$, the frequency of reflected sound as heard by driver is .... $Hz$
AIPMT 2009, Medium
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Car is the source and the hill is observer.
Frequeney heard at the hill, $v_{1}$
$\therefore v_{1}=\frac{v \times v}{(v-V)}=\frac{600 \times 330}{330-30}$
Now for reflection, the hill is the source and the driver the observer.
$\therefore v_{2}=v_{1} \times \frac{(330+30)}{330}$
$\Rightarrow v_{2}=\frac{600 \times 330}{300} \times \frac{360}{330} \Rightarrow v_{2}=720 \mathrm{Hz}$
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