First slide

Doppler

Question

A train is passing by a platform at a constant speed of 40 m/s. The train horn is sounded at a frequency of 378 Hz. Find the overall change in frequency as detected by a person standing on the platform, as the train moves from approaching to receding. (Take velocity of sound in air as 320 m/s)

Moderate

A

104 Hz
B

92 Hz
C

84 Hz
D

96 Hz

Solution

For situation l, $f_{ap 1} = \frac{v - 0}{v - v_{s}} \times f$
= $\frac{320}{320 - 40} \times 378 = 432 Hz$

For situation 2, $f_{{ap}_{2}} = \frac{v}{v - (- v_{S})} \times f = 336 Hz$
$f = f_{ap 1} - f_{ap 2} = 96 Hz$

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