The apparent frequency of the whistle of an engine changes by the ratio 5/3 as the engine passes a stationary observer. If the velocity of sound is 340 ms−1, then the velocity of the engine is
A
340 ms−1
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B
170 ms−1
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C
85 ms−1
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D
42.5 ms−1
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Solution
The correct option is D 85 ms−1
Given : Velocity of sound vsound=340m/s
Let the frequency of the whistle be n and velocity of the engine be v.
Let the apparent frequencies heard by the observer be n′ and n′′ before and after the source passes the observer.
Doppler effect when source (S′) approaches the stationary observer:
n′=n[vsoundvsound−vsource]=n[340340−v]
Doppler effect when source S moves away from the stationary observer: