The frequency of echo will be $.......Hz$ if the train blowing a whistle of frequency $320\,Hz$ is moving with a velocity of $36\,km / h$ towards a hill from which an echo is heard by the train driver. Velocity of sound in air is $330\,m / s$.
JEE MAIN 2022, Diffcult
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The hill will be a secondary source.
$f _{1}=$ frequency of the car w.r.t. the hill
$f_{1}=\left(\frac{v}{v-v_{ s }}\right) f=\left(\frac{330}{320}\right) \times 320=330 Hz$
$f _{2}=$ Frequency of the sound reflected by hill w.r.t. the car $(echo)$
$f_{2}=\left(\frac{v+v_{0}}{v}\right) f_{1}=\frac{(330+10)}{330} \times 330=340\,Hz$
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