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Question:

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

20/19

18/17

19/18

21/20

Solution:

The correct option is B
19/18
f_app = f_0[v_2 ± v_0 / v_2 ∓ v_s]
f1 = f0[340 / 340 - 34]
f2 = f0[340 / 340 - 17]
f1/f2 = 340 - 17 / 340 - 34 = 323 / 306 => f1/f2 = 19/18.