Question

The train is approaching station with 72 km$h^{-1}$. When 1 km away it blows a whistle of frequency 600 Hz. The frequency heard by the person is $(v_{sound}=350m{s}^{-1})$

• A. 612 Hz
• B. 625 Hz
• C. 636 Hz
• D. none of these

Answer 1

The correct option is C 636 Hz

Let the original frequency of the source $f_o=600Hz$.
Velocity of train $V_{train}=72\times \frac{5}{18}=20m/s$
Using Doppler effect when a source is moving towards stationary observer,
Apparent frequency heard $f=f_o[\frac{V_{sound}}{V_{sound}-V_{train}}]$
$\therefore$ $f=600[\frac{350}{350-20}]$
$⟹f=636Hz$
So, option C is correct.