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Riddhi Chia
Asked: 2022-11-11T03:09:56+05:30 In:

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%`

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%`
Parabola
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  1. Correct Answer – B
    `f_(“before crossing”) = f_(0)((c)/(c – v_(S))) = 1000((320)/(320 – 20))`
    `f_(“after crossing”) = f_(0)((c)/(c + v_(S))) = 1000((320)/(320 + 20))`
    `Deltaf = f_(0)”((2cv_(S))/(c^(2) – v_(S)^(2))) , (Deltaf)/(f) xx 100% = (2 xx 320 xx 20)/(300 xx 340) xx 100 = 12.54% approx 12%`

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Shobha Nayak
Asked: 2022-11-09T18:31:36+05:30 In:

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%`

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%`
Parabola
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1 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%`

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