Q128 / 160 · WBJEE 2012 · Physics
A train approaching a railway platform with a speed of $20 \mathrm{ms}^{-1}$ starts blowing the whistle. Speed of sound in air is $340 \mathrm{ms}^{-1}$. If the frequency of the emitted sound from the whistle is $640 \mathrm{Hz}$, the frequency of sound to a person standing on the platform will appear to be
- A$600 \mathrm{Hz}$
- B$640 \mathrm{Hz}$
- Correct answerC$680 \mathrm{Hz}$
- D$720 \mathrm{Hz}$
Solution
Given $\quad v=340 \mathrm{ms}^{-1}, \quad u_{5}=20 \mathrm{ms}^{-1} \quad$ and
$v_{0}=640 \mathrm{Hz}$
From Doppler's law,
$$
\begin{array}{l}
v=\left(\frac{340}{340-20}\right) 640 \\
v=680 \mathrm{Hz}
\end{array}
$$
Hence, the correct answer is option: 'C'.