Question

If to a stationary observer the apparent frequency appear to be 6/5 times the actual frequency of the moving source , then the speed and direction of the sound source is : (velocity of sound = 360 m/sec)

• A. 60 m/s towards the observer
• B. 60 m/s away from the observer
• C. 55 m/s towards the observer
• D. 55 m/s away from the observer

Answer 1

Answer: (A)

60 m/s towards the observer

Apparent frequency, $f^{\prime }=\left(\frac{V}{V\pm V_s}\right)f$
$\Rightarrow \frac{6f}{5}=\left(\frac{V}{V-V_s}\right)f$

As $f^{\prime }>f\Rightarrow$ source is moving towards observer.

$\frac{6}{5}=\left(\frac{360}{360-V}\right)$

$\Rightarrow V=60m{s}^{-1}$