A sounding source of frequency 500 Hz moves towards a stationary observer with a velocity $30 \mathrm{~m} / \mathrm{s}$. If the velocity of sound in air is $330 \mathrm{~m} / \mathrm{s}$, find the frequency heared by the observer.

• A

  500 Hz
• Correct answer
  B

  550 Hz
• C

  355 Hz
• D

  55.5 Hz

Solution

Here, $n=500 \mathrm{~Hz}, v_o=0$ $v_s=30 \mathrm{~m} / \mathrm{s}, v=330 \mathrm{~m} / \mathrm{s}$ $\therefore$ From, $n^{\prime}=n\left(\frac{v-v_o}{v-v_s}\right)$ $\begin{aligned} & =500\left(\frac{330}{330-30}\right) \\ & =550 \mathrm{~Hz}\end{aligned}$

Hence, the correct answer is option: 'B'.