Two trains `A and B` moving with speeds `20m//s` and `30m//s` respectively in the same direction on the same straight track, with `B` ahead of `A`. The engines are at the front ends. The engine 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 thus `320 Hz`. the speed of sound in still air is `340 m//s`.
(5) The distribution of the sound intensity of the whistle as observed by the passengers in train `A` is best represented by

Options

A.
B.
C.
D.

Answer

A

Explanation

Because
`V_(S) = V_(O) = 20`
so as seen form passengers of train `A`,
`f' = f_(0)`