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 \(...Hz\).

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Answer & Solution

Correct Answer

(C) \(20\)

Step-by-step Solution

Detailed explanation

\(V_{S}=0, V_{o b}=5 m / s\)

\(f _{\text {direct }}=\left(\frac{320-5}{320}\right) 640=630 Hz\)

\(f _{\text {reflected }}=\left(\frac{320+5}{320}\right) 640=650 Hz\)

\(f _{\text {beat }}=650-630=20 Hz\)

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