Question

A train moves towards a stationary observer with speed 34m/s. The train sounds a whistle and its frequency registered by the observer is f1. If the train’s speed is reduced to 17m/s, the frequency registered is f2. If the speed of sound is 340m/s, then the ratio f1/f2 is?

(a) 18/19

(b) 1/2

(c) 2

(d) 19/18

I got this question in an interview.

This intriguing question comes from Transverse Waves topic in section Waves of Engineering Physics I

Answer 1

Correct option is (d) 19/18

Easiest explanation: For the stationary observer,

f^‘=v/(v-vs)×f

f1=340/(340-34)×f

f2=340/(340-17)×f

f1/f2 =(340-17)/(340-34)=19/18.