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Question

A train moves towards a stationary observer with a speed of 34 m/s. The train sounds a whistle and its frequency registered by the observer is f1 . If the train speed is reduced to 17 m/s, the frequency registered is f2. If the speed of sound is 340 m/s, then the ratio f1f2 is

A
12
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B
2
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C
1819
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D
1918
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Solution

The correct option is D 1918
Let f be the actual frequency
Case 1:
Speed of observer, v0=0
Speed of source, vs=34 m/s
Apparent frequency observed by observer,
f1=(v±v0v±vs)f
f1=(34034034)f
f1=109f

Case 2:
Speed of observer, v0=0
Speed of source, vs=17 m/s
Apparent frequency observed by observer,
f2=(v±v0v±vs)f
f2=(34034017)f
f2=2019f
f1f2=1918

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