A train moving at 40 m/s, passes by a stationary observer, emitting a whistle of frequency 300 Hz. If the velocity of sound wave is 340 m/s, then the change in the apparent frequency of the sound, just before and after the train passes by the observer will be nearly

Options

A. 32 Hz
B. 72 Hz
C. 40 Hz
D. 8 Hz

Answer

B

Explanation

`n_(1) = n [(v)/(v-v_(s))] and n_(2) = n [(v)/(v + v_(s))]`
`n' = n_(1) - n_(2)`
`n' = nv [(1)/(v- v_(s)) - (1)/(v + v_(s))]`
`:. n' = nv [(v + v_(s) - v + v_(s))/((v - v_(s)) (v + v_(s)))]`
`= (300 xx 340 xx 2 xx 40)/((340 - 40) (340 + 40))`
`= (300 xx 340 xx 2 xx 40)/(300 xx 380)`
`= (4 xx 2 xx 40)/(38) ~~72 Hz`