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A train moving at 34 m/s towards a stationary observer sounds its whistle; the observer records frequency f1. The train then slows to 17 m/s and the observed frequency is f2. If the speed of sound is 340 m/s, find the ratio f1/f2.

1. 18/19
2. 1/2
3. 2
4. 19/18

Correct answer: 19/18

Solution

For source moving towards stationary observer: f_obs = f0 * v/(v - vₛ). f1 = f0*340/(340-34) = f0*340/306. f2 = f0*340/(340-17) = f0*340/323. f1/f2 = 323/306 = 17*19/(17*18) = 19/18.

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