Two cars $A$ and $B$ are moving away from each other in opposite directions. Both the cars are moving with a speed of $20\, ms^{-1}$ with respect to the ground. If an observer in car $A$ detects a frequency $2000\, Hz$ of the sound coming from car $B$, what is the natural frequency of the sound source of car $B$ .... $Hz$ ? (speed of sound in air $= 340\, ms^{-1}$)
JEE MAIN 2019, Medium
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Doppler effect:
$f=\left(\frac{v+u_{0}}{v-u_{s}}\right)\left(f_{0}\right)$
$2000=\left(\frac{340+(-20)}{340-(-20)}\right)\left(f_{0}\right)$
$\mathrm{f}_{\mathrm{o}}=2250 \mathrm{Hz}$
$f=\left(\frac{v+u_{0}}{v-u_{s}}\right)\left(f_{0}\right)$
$2000=\left(\frac{340+(-20)}{340-(-20)}\right)\left(f_{0}\right)$
$\mathrm{f}_{\mathrm{o}}=2250 \mathrm{Hz}$
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