1.

A train is moving on a straight track with speed 20 ms^(-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 :

Answer»

`12%`
0.18
0.24
0.06

Solution :`f_(1)= F[ (v)/(v-v_(2)) ]= f [ (320)/(320 - 20) ]= f xx (320)/(300) `Hz
` f_(2) = f [ (v)/(v + v_(s)) ]= f xx (320)/(340)` Hz
100 `xx (f_(2)/(f_(1)) - 1) = ((f_(2) - f_(1))/(f_(1))) xx 100 `
= 100` [(300)/(340) - 1 ] ` = 12 %
So correct CHOICE is a.


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