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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%` |
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Answer» 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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