A source of sound is travelling towards a stationary observer. The frequency of sound heard by the observer…

A source of sound is travelling towards a stationary observer. The frequency of sound heard by the observer is three times the original frequency. The velocity of sound is $v\text{ ms}^{-1}$. The speed of source will be
  1. $\frac{2}{3} v$
  2. $v$
  3. $\frac{3}{2} v$
  4. $3v$

Solution

$f' = f \left(\frac{v}{v - v_s}\right) \implies \frac{f'}{f} = \frac{v}{v - v_s}$ $\implies \frac{v}{v - v_s} = 3 \implies v_s = \frac{2v}{3}$

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