A siren emitting sound of frequency $800 \mathrm{~Hz}$ is going away from a static listener with a speed of $30 \mathrm{~m} / \mathrm{s}$. The frequency of sound heard by listener is (velocity of sound $=300 \mathrm{~m} / \mathrm{s}$ )

Question

A siren emitting sound of frequency $800 \mathrm{~Hz}$ is going away from a static listener with a speed of $30 \mathrm{~m} / \mathrm{s}$. The frequency of sound heard by listener is (velocity of sound $=300 \mathrm{~m} / \mathrm{s}$ ), $727.3 \mathrm{~Hz}$, $481.2 \mathrm{~Hz}$, $644.8 \mathrm{~Hz}$, $286.5 \mathrm{~Hz}$

MARKS App · Accepted Answer

Here, $v=800 \mathrm{~Hz}, v_s=30 \mathrm{~m} / \mathrm{s}, v=300 \mathrm{~m} / \mathrm{s}$. As the source is going away from the stationary observer, therefore, $v^{\prime}=\frac{v \times v}{v+v_s}=\frac{300 \times 800}{300+30}=\frac{300 \times 800}{330}=727.3 \mathrm{~Hz} .$

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