Question

A siren placed at a railway platform is emitting sound of frequency 5 kHz. A passenger sitting in a moving train A records a frequency of 5.5 kHz while the train approaches the siren. During his return journey in a different train B he records a frequency of 6.0 kHz while approaching the same siren. The ratio of the velocity of train B to that of train A is

• A. 242/252
• B. 2
• C. 5/6
• D. 11/6

Answer 1

The correct option is B 2

In both the cases observer is moving towards the source.Hence by using $n^{\prime }=n\left(\frac{v+v_0}{v}\right)$


When passenger is sitting in train A, then
$5.5=5\left(\frac{v+v_A}{v}\right)$ ....(i)

When passenger is sitting in train B, then
$6=5\left(\frac{v+v_B}{v}\right)$ ....(ii)

On solving equation (i) and (ii) we get $\frac{v_B}{v_A}=2$