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 speed of train is reduced to 17 m/s, the frequency registered is `f_(2)`. If speed fo sound is 340 m/s, then the ratio `f_(1)//f_(2)` is :

Options

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

Answer

B

Explanation

The general formula for the apparent frequency is
`n_(a) = ((v _ v_(L))/(v - v_(S)))n`
In this case the observer is at rest, `:. v_(L) = 0`
`:. f_(1) = ((v)/(v - v_(S))) f. = ((340)/(340 - 34)) f = (340)/(306)`
and `f_(2) = ((340)/(340 - 17)) f = (340)/(323)`
`:. (f_(1))/(f_(2)) = (340)/(306) xx (323)/(340) = (323)/(306) = (17 xx 19)/(17 xx 18) = (19)/(18)`