Question

A siren emits sound of frequency $1000\, Hz$, it moves away from the observer towards a cliff with a speed of $ 10\,ms^{-1}$ . Then, the frequency of sound heard by observer coming directly from the siren is

• A. less than 1000 Hz
• B. greater than 1000 Hz
• C. equal to 1000 Hz
• D. no sound is heard

Answer 1

Answer: (A)

Sound heard
$
\begin{array}{l}
f _{1}= f _{0}\left(\frac{ v }{ v + v _{ s }}\right) \\
v _{ s }=10 ms ^{-1} \\
f _{1}=\left(\frac{330}{330+10}\right) \times 1000 \\
f _{1}=\frac{33}{34} \times 1000 \text { is less than } 1000 Hz
\end{array}
$