A train is moving on a straight track with speed 20 ms–1. It is blowing its whistle at the frequency of 1000 Hz. The percentage change in the frequency heard by a person standing near the track as the train passes him is (speed of sound = 320 ms

Engineering

Physics

Dopplers Effect

Question

A train is moving on a straight track with speed 20 ms^–1. It is blowing its whistle at the frequency of 1000 Hz. The percentage change in the frequency heard by a person standing near the track as the train passes him is (speed of sound = 320 ms^–1) close to :

24%

18%

12%

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$f_{1} = f_{0} (\frac{320}{320 - 20}) = f_{0} \frac{320}{300}$

$f_{2} = f_{0} (\frac{320}{320 + 20}) = f_{0} \frac{320}{340}$

$\frac{f_{1} - f_{2}}{f_{0}} \times 100 = (\frac{320}{300} - \frac{320}{340}) \times 100$

= (1.066 – 0.941) × 100 $≅$ 12.4%