Question

A siren placed at a railway platform is emitting sound of frequency 5kHz. 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 6kHz while approaching the same siren. The ratio of the velocity of train B to that train A is?

(a) 242/252

(b) 2

(c) 5/6

(d) 11/6

Answer 1

Right option is (b) 2

The best explanation: When the observer approaches stationary source,

γ^‘=(v+v0)/v×γ

For train A, 5.5=(v+vA)/v×5 or vA=0.1v

For train B, 6=(v+vB)/v×5 or vB=0.2v

vB/vA = 2.