Question

A source of frequency $240\, Hz$ is moving towards an observer with a velocity of $20\, m/s$. The observer is now moving away from the source with a velocity of $20\, m/s$. Apparent frequency heard by observer, if velocity of sound is $340\, m/s$, is

• A. 268 Hz
• B. 270 Hz
• C. 360 Hz
• D. 240 Hz.

Answer 1

Answer: (D)

240 Hz.

Answer 2

The frequency of the source $v_{0}=240 \,Hz$ The velocity of the source $u _{ s }=+20\, m / s$ (as it moves towards the observer) The velocity of the observer $u _{0}=-20 \,m / s$ (as it moves away from the source) Applying Doppler's effect $v=v_{0}\left[\frac{v+u_{0}}{v-u_{s}}\right]$ $=240 \times\left[\frac{340+(-20)}{340-20}\right]=240\, Hz$