A policeman on duty detects a drop of $10\%$ in the pitch of the horn of a moving car as it crosses him. If the velocity of sound is $330 \,m/s,$ the speed of the car will be ..... $m/s$
Diffcult
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$f_{1} / f_{2}=\left[\frac{c}{c-v} / \frac{c}{c+v}\right]$
$\Rightarrow \frac{f_{1}}{f_{2}}=\frac{c+v}{c-v} \Rightarrow \frac{100}{90}=\frac{c+v}{c-v}$
$\Rightarrow 100 c-100 v=90 C+90 v$
$\Rightarrow 10 c=190 v$
$\Rightarrow 1 / 19 \times 330=17.3 \mathrm{m} / \mathrm{s}$
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