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A whistle revolves in a circle with angular speed $\omega=20 \mathrm{rad} \mathrm{s}^{-1}$ 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 away from the centre $\left(v_{\text {sound }}=340 \mathrm{~ms}^{-1}\right)$ ?
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Verified Answer
The correct answer is:
$375 \mathrm{~Hz}$
Velocity of source is given as
$$
\begin{gathered}
V_{\text {source }}=\text { rus }=0.5 \times 20 \\
=10 \mathrm{~ms}^{-1}
\end{gathered}
$$
$\therefore$ Minimum frequency is $f \mathrm{~min}$
$$
\begin{aligned}
& =\left(\frac{V}{V+V_s}\right) f \\
& =\frac{340}{340+10} \times 385=374 \mathrm{~Hz}
\end{aligned}
$$
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