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

C

Explanation

When an source approaches a stationary observer, the frequency heard by the observer is `upsilon=upsilon_(0)((v)/(v-v_(s)))`
where, `upsilon_(0)` = source frequency
v = speed of sound
`v_(s)` = speed of source
As per question
`upsilon_(1)=(upsilon_(0)340)/((340-34))andupsilon_(2)=(upsilon_(0)340)/((340-17))`
`therefore (upsilon_(1))/(upsilon_(2))=((340-17))/((340-34))=(323)/(306)=(19)/(18)`