Question

A train approaching a railway platform with a speed of $20m/s$ starts blowing a whistle. Speed of sound in air is $340m/s$. If the frequency of emitted sound from the whistle is $640Hz$, the frequency of sound to a person standing on the platform will appear to be (in $Hz$)

• A. $600$
  
• B. $640$
  
• C. $680$
  
• D. $720$
  

Answer 1

The correct option is C $680$

The apparent frequency of sound , observed by a stationary listener , when a source of sound is approaching to listener , is given by

${\nu }^{\prime }=\nu \left(\frac{v}{v-v_s}\right)$

Given - $\nu =640Hz,v=340m/s,v_s=20m/s$

Hence ${\nu }^{\prime }=640\left(\frac{340}{340-20}\right)=\frac{640\times 340}{320}$

or ${\nu }^{\prime }=680Hz$