A train is moving on a straight track with speed $20\ ms^{-1}$. It is blowing its whistle at the frequency of $1000\ Hz$. The percentage change in the frequency heard by a person standing near the track as the train passes him is ( speed of sound $=320$ $ms^{-1}$ ) close to .... $\%$
JEE MAIN 2015, Medium
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$f_{1}=f\left[\frac{v}{v-v_{s}}\right]=f \times \frac{320}{300} \mathrm{Hz}$
$f_{2}=f\left[\frac{v}{v+v_{s}}\right]=f \times \frac{320}{340} \mathrm{Hz}$
$\left(\frac{f_{2}}{f_{1}}-1\right) \times 100=\left(\frac{300}{340}-1\right) \times 100 \simeq 12 \%$
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