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A train is moving on a straight track wi...

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

`6%`

B

`12%`

C

`18%`

D

`24%`

Text Solution

Verified by Experts

The correct Answer is:
B
Before `f.=f(v/(v-v_s))impliesf.=1000((320)/(320 -20))`
`implies f. = 1000 xx 320/300 = (3200)/3`
After `f..=f(v/(v+v_s))=1000((320)/(320+20))=(320000)/34`
change in frequency
`Deltaf=f-f..= (3200)/3 - (32000)/(34)`
`= 1066.67 -941.18 - 125.49`
% Change `=(125.49)/(1066.67)xx100`
`=11.76% ~~ 12%.`
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