A train moves towards a stationary observer with speed `34 m//s`. The train sounds a whistle and its frequency registered by the observer is `f_(1)`. If the train's speed is reduced to `17 m//s`, the frequency registered is `f_(2)`. If the speed of sound of `340 m//s`, then the ratio `f_(1)//f_(2)`is

Options

A. `18//19`
B. `1//2`
C. `2`
D. `19//18`

Answer

D

Explanation

`{:(rarr v_(s) = 34m//s,,rarr v_(o) = 0),(overset(*)(S)" "overset(v)rarr,,overset(*)(O)):}`
`f_(1) = f((v)/(v-v_(s))) = f((340)/(340-34)) = (340)/(323)f`
`f_(2)=f (340/(340-17))=340/323 f`
`(f_(1))/(f_(2)) = (323)/(306) = (19)/(18)`