A policemen buzz a whistle of frequency $400\ Hz$. A car driver is approaching the policemen. The speed of car is $54\ kmh^{-1}$. The change in frequency experienced by the driver, when driver approaches the policemen and after he crosses the policemen, is ... $Hz$ [Velocity of sound is $350\ ms^{-1}$]
Diffcult
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When driver approaches to the policeman then observed frequency $n^{\prime}=n\left(\frac{v+v_{0}}{v}\right)$
after crossing $n^{\prime \prime}=n\left(\frac{v-v_{0}}{v}\right)$ where $n=400 \mathrm{\,Hz},$
$\mathrm{v}_{0}=54 \mathrm{\,km} / \mathrm{hr}=15 \mathrm{\,m} / \mathrm{s}$
their difference $\Delta n=n^{\prime}-n^{\prime \prime}=n\left(\frac{v+v_{0}}{v}\right)-n$
$\left(\frac{v-v_{0}}{v}\right)=\frac{2 n v_{0}}{v}=\frac{2 \times 400 \times 15}{350}=34.2 \mathrm{\,Hz}$
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