Question

A train is moving on a straight track with speed `20ms^(-1)`. It is blowing its whistle at the frequency of `1000 Hz`. The percentage change in the frequency heard by a person standing near the track as the train passes him is (speed of sound `= 320 ms^(-1)`) close to :
A. `12%`
B. `6%`
C. `18%`
D. `24%`

Answer 1

Correct Answer - A
Observer is stationary and source is moving.
During approach, `f_(1) = f((nu)/(nu - nu_(s)))`
`= 1000((320)/(320 - 20))`
`= 1066.67 H_(Z)`
During recede, `f_(2) = f((nu)/(nu + nu_(s)))`
`= 1000((320)/(320 + 20)) = 941.18 H_(Z)`
`|%` change in frequency| `= ((f_(1) - f_(2))/(f_(1))) xx 100`
`~~ 12%`