A whistle revolves in a circle with an angular speed of $20\; rad/sec$ using a string of length $50 \;cm.$ If the frequency of sound from the whistle is $385\; Hz,$ then what is the minimum frequency heard by an observer, which is far away from the centre in the same plane ... $Hz$ ? ($v = 340 \;m/s$)
AIPMT 2002, Medium
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(b) Minimum frequency will be heard, when whistle moves away from the listener.
${n_{\min }} = n\,\left( {\frac{v}{{v + {v_s}}}} \right)$ where $v = r\omega = 0.5 \times 10 = 1\,m/s$
==> ${n_{\min }} = 385\,\left( {\frac{{340}}{{340 + 10}}} \right) = 374Hz.$
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