Two trains A and B are moving with speeds 20 m/s and 30 m/s respectively in the same direction on the same straight track, with B ahead of A. The engines are at the front ends. The engines of train A blows a long whistle.
Assume that the sound of the whistle is composed of components varying in frequency from f 1 = 800 Hz to f 2 = 1120 Hz, as shown in the figure. The spread in the frequency (highest frequency – lowest frequency) is 320 Hz. The speed of sound in still air is 340 m/s.
(i) The speed of sound of the whistle is
Text Solution
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(i)
Sol. V S/A = 340 + 20 = 360
V S/B = 340 – 30 = 310
(ii)
Sol. Because
V S = V O = 20
so as seen from passengers of train A,
f´ = f 0
(iii)
Sol. As seen from B
f´ min = 
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