A train moves towards a stationary observer with a speed of 40 m/s. The observer records the frequency of the train's whistle as `f_(1)`. But when the train's speed is reduced to 20 m/s, the frequency of the whistle recored by him is `f_(2)`. What is the ratio `(f_(1))/(f_(2))`, if the speed of sound in air is 340 m/s ?

Options

A. `(17)/(16)`
B. `(16)/(15)`
C. `(15)/(14)`
D. `(15)/(16)`

Answer

B

Explanation

In both cases, the train moves towards the observer
`f_(1) = ((v)/(v - v_(s))) xx n = ((340)/(340 - 40)) xxn`
` = ((340)/(300))n`
and `f_(2) = ((340)/(340 - 20)) xxn = ((340)/(320))n`
`:. (f_(1))/(f_(2)) = (320)/(300) = (16)/(15)`