Two cars ${X}$ and ${Y}$ are approaching each other with velocities $36\; {km} / {h}$ and $72\; {km} / {h}$ respectively. The frequency of a whistle sound as emitted by a passenger in car ${X}$, heard by the passenger in car ${Y}$ is $1320 \;{Hz}$. If the velocity of sound in air is $340\; {m} / {s}$, the actual frequency of the whistle sound produced is ........ $Hz.$
JEE MAIN 2021, Medium
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${V}_{{x}}=36 {km} / {hr}=10 {m} / {s}$
${V}_{{y}}=72 {km} / {hr}=20 {m} / {s}$
by doppler's effect
${F}^{\prime}={F}_{0}\left(\frac{{V} \pm {V}_{0}}{{V} \pm {V}_{{s}}}\right)$
$1320={F}_{0}\left(\frac{340+20}{340-10}\right) \Rightarrow {F}_{0}=1210 {Hz}$
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