A whistle with frequency 1020 Hz is blown at a station. A man travelling in train moving towards the station…

A whistle with frequency 1020 Hz is blown at a station. A man travelling in train moving towards the station at $30\text{ ms}^{-1}$ hears the sound of the whistle. If the speed of sound is $340\text{ ms}^{-1}$, the apparent frequency heard by him is
  1. 1020 Hz
  2. 1110 Hz
  3. 2040 Hz
  4. 610 Hz

Solution

Apparent frequency $n' = n\left(\frac{v + v_o}{v}\right)$ $\Rightarrow n' = 1020\left(\frac{340 + 30}{340}\right)$ $\Rightarrow n' = 1020 \times 1.088 \Rightarrow n' = 1110\text{ Hz}$

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