The frequency of a car horn encountered a change from $400\, {Hz}$ to $500\, {Hz}$, when the car approaches a vertical wall. If the speed of sound is $330\, {m} / {s}$. Then the speed of car is $.....\,{km} / {h} .$
JEE MAIN 2021, Diffcult
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Wall as an observer
Frequency received by wall
${f}_{1}={f}_{0}\left(\frac{{C}}{{C}-{V}}\right)$
Again wall as a source
Frequency received by observer on car
${f}_{2}={f}_{1}\left(\frac{{C}+{V}}{{C}}\right)$
$500=400\left(\frac{{C}+{v}}{{C}-{V}}\right)$
$\frac{5}{4}=\frac{{C}+{V}}{{C}-{V}}$
${C}=9 {V}$
${V}=\frac{{C}}{9}=\frac{330}{9}\,{m} / {s}$
${V}=\frac{330}{9} \times \frac{18}{5}=132\, {km} / {hr}$
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