Question

A train is approaching a station with a uniform velocity of 72 kmph and the frequency of the whistle of that train is 480 Hz. The apparent increase in the frequency of that whistle heard by a stationary observer on the platform is (Velocity of sound in air is 340m/s)

• A. 60 Hz
• B. 45 Hz
• C. 30 Hz
• D. 15 Hz

Answer 1

The correct option is C 30 Hz

Given : $v_s=72km/h=72\times \frac{1000}{3600}=20m/s$

$\delta =480H_z$

$v=340m/s$

By doppler effect formula

${\delta }^{{}^{\prime }}=\delta \left(\frac{v-v_0}{v-v_3}\right)$

$=480\left(\frac{340-0}{340-20}\right)$

$=\frac{480\times 340}{320}$

$=510H_z$

So, ${\delta }^{{}^{\prime }}-\delta =510-480=30H_z$

So, apparent increase in frequency is 30 $H_z$