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. (a) `18%`
B. (b) `24%`
C. ( c) `6%`
D. (d) `12%`

Answer 1

Correct Answer - D
(d) `f_(1) = f[(v)/(v + v_(s))] = f xx (320)/(300) Hz`
`f_(2) = f[(v)/(v + v_(s))] = f xx (320)/(340) Hz`
`((f_(2))/(f_(1)) -1) xx 100 = ((300)/(340) - 1) xx 100 ~= 12%`