Two trains move towards each other with same speed. Speed of sound is `340 m//s`. If the pitch of the tone of the whistle of one train when heard on the other train changes to `9//8` times, then the speed of each train is :

Options

A. `20 m//s`
B. `2 m//s`
C. `200 m//s`
D. `2000 m//s`

Answer

A

Explanation

Here `v.=9/8 v`
Source and observer are moving in opposite direction, therefore apparent frequency
`v.=vtimes(v+u)/(v-u)implies9/8v=vtimes(340+u)/(340-u)`
`impliesu=340/17=20m//sec`