A whistle revolves in a circle with an angular speed of 20 rad / … - Vidyadip

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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