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Question

The frequency of the whistle of an engine appears to be (56)th of the original frequency , when it passes a stationary observer. If the speed of sound in air is 350 m/s, then the speed of the engine is:

[Assume, medium is stationary and the original frequency of the whistle is measured when the source and observer are at rest]

A
35 m/s
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B
70 m/s
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C
105 m/s
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D
140 m/s
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Solution

The correct option is B 70 m/s
Given,
Speed of sound in stationary air medium, vs=350 m/s

Let the original frequency of the whistle be f0 and the speed of the engine be v when it passes a stationary observer.

Frequency heard by the stationary observer, fapp=56f0
fapp<f0

So, we can say that, source of sound is moving away from the observer.

Now, the apparent frequency heard by the observer, fapp=(vv+vs)f0

56f0=f0[350350+v]

5×350+5v=6×350v=3505

v=70 m/s

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