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 :

Options

A. 0.06
B. 0.12
C. 0.18
D. 0.24

Answer

B

Explanation

`f_(1) = f[(v)/(v-v_(s))]=f[(320)/(320 - 20)] = f xx (320)/(300) Hz`[Coming towards observer]
`f_(2) = f[(v)/(v-v_(s))]=f xx (320)/(340) Hz` [moving away from observer]
`% `change `= ((f_(2) - F_(1)/(f_(1))) xx 100 = 100 [(300)/(340) - 1 ] = 12 %`