WBJEE · Physics · Waves and Sound
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 \(640 \mathrm{Hz}\)
- C \(680 \mathrm{Hz}\)
- D \(720 \mathrm{Hz}\)
Answer & Solution
Correct Answer
(C) \(680 \mathrm{Hz}\)
Step-by-step Solution
Detailed explanation
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, \[ \begin{array}{l} v=\left(\frac{340}{340-20}\right) 640 \\ v=680 \mathrm{Hz} \end{array} \]
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