A train moving at a speed of `220ms^-1` towards a stationary object emits a sound of frequency 1000 Hz. Some of the sound reaching the object gets reflected back to the train as echo. The frequency of the echo as detected by the driver of the train is (speed of sound in air is `330ms^(-1)`)

Options

A. `3500Hz`
B. `4000Hz`
C. `5000Hz`
D. `3000Hz`

Answer

C

Explanation

Here, `upsilon_(s)=220m//s, upsilon_(L)=0, v=1000Hz `
`upsilon=330m//s`
If`v^(')` is frequency of sound striking the stationary object, then `v^(')=(upsilonxxv)/(upsilon-upsilon_(s))=(330xx1000)/(330-220)=3000Hz`
For the reflected sound, `upsilon_(s)=0, upsilon_(L)=220m//s`
If `v^('')` is frequency of reflected sound as perceived by the person in moving train, then
`v^('')=((upsilon+upsilon_(L))v^('))/(upsilon)=((330+220)3000)/(330)=5000Hz`