Two trains `A` and `B` are moving with speeds `20 m//s` respectively in the same direction on the same stright 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 figure. The speed in the frequency (highest frequency - lowest frequency) is thus `320 Hz`. The speed of sound in still air is `340 m//s`.

The spread of frequency as observed by the passengers in train `B` is

Options

A. 310Hz
B. 330Hz
C. 350Hz
D. 290Hz

Answer

A

Explanation

For the passengers is train B,oberever is recending with vlocity `30m//s` and source is approaching with velocity `20 m//s`
`f_1=800((340-30)/(340-20))=775 Hz`
`f_1=1120((340-30)/(340-20))=1085 Hz`
Therefore spread of frequency ,`f'_2-f'_1=310 Hz`