Question

A whistle giving out 450 Hz approaches a stationary observer at a speed of 33 m$s^{-1}$ and velocity of sound is 330m/s The frequency heard by the observer in Hz is

• A. 409
• B. 429
• C. 517
• D. 500

Answer 1

The correct option is D 500

Velocity of sound $v_{sound}=330m/s$

Speed of source $v_{source}=33m/s$

Frequency of source $\nu =450Hz$

Doppler effect when source moves towards the stationary observer :

Apparent frequency heard by the observer ${\nu }^{\prime }=\nu [\frac{v_{sound}}{v_{sound}-v_{source}}]$

${\nu }^{\prime }=450[\frac{330}{330-33}]=500Hz$