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Two sound sources, each producing waves of wavelength λ, are held at a fixed separation. An observer travels with speed u along the straight line joining the two sources. The beat frequency heard by the observer per second is

1. u/2λ
2. 2u/λ
3. u/λ
4. u/3λ

Correct answer: 2u/λ

Solution

Moving toward one source gives f1 = (v+u)/lambda; moving away from the other gives f2 = (v-u)/lambda. Beat frequency = f1 - f2 = 2u/lambda.

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