An observer is riding on a bicycle and moving towards a hill at $18\,km\,h ^{-1}$. He hears a sound from a source at some distance behind him directly as well as after its reflection from the hill. If the original frequency of the sound as emitted by source is $640\,\,Hz$ and velocity of the sound in air is $320\,m / s$, the beat frequency between the two sounds heard by observer will be $...Hz$.
JEE MAIN 2022, Diffcult
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$V_{S}=0, V_{o b}=5 m / s$
$f _{\text {direct }}=\left(\frac{320-5}{320}\right) 640=630 Hz$
$f _{\text {reflected }}=\left(\frac{320+5}{320}\right) 640=650 Hz$
$f _{\text {beat }}=650-630=20 Hz$
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