JEE Main 2015 · 4 Apr · Shift 1 · Physics Q13

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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 :

12%

18%

24%

Answer and worked solution

Official answer: B

Approaching: $f_1=1000\cdot\frac{320}{300}=1066.7$ Hz. Receding: $f_2=1000\cdot\frac{320}{340}=941.2$ Hz.
$\frac{f_1-f_2}{f_1}=\frac{125.5}{1066.7}\approx12\%.$

Topics:Doppler effect

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JEE Main 2015 · 4 Apr · Shift 1, Physics question 13