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/sGiven, 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=6×350⇒v=3505 ⇒v=70 m/s

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