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Waves and Sound — JEE physics STD 12 Science — Question

CBSE BoardEnglish MediumSTD 12 ScienceJEE physicsWaves and Sound1 Mark
MCQ
A whistle revolves in a circle with an angular speed of $20 \mathrm{rad} / \mathrm{sec}$ using a string of length $50 \mathrm{~cm}$. If the frequency of sound from the whistle is $385 \mathrm{~Hz}$, then what is the minimum frequency heard by an observer, which is far away from the centre in the same plane ? ( $v=$ $340 \mathrm{~m} / \mathrm{s})$
  • A
    $333 \mathrm{~Hz}$
  • ✓
    $374 \mathrm{~Hz}$
  • C
    $385 \mathrm{~Hz}$
  • D
    $394 \mathrm{~Hz}$
✓

Answer

Correct option: B.
$374 \mathrm{~Hz}$
(b) Minimum frequency will be heard, when whistle moves away from the listener.
$n_{\min }=n\left(\frac{v}{v+v_s}\right) \text { where } v=r \omega=0.5 \times 10=1 \mathrm{~m} / \mathrm{s} $
$\Rightarrow n_{\min }=385\left(\frac{340}{340+10}\right)=374 \mathrm{~Hz} .$

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