A sound wave of wavelength ` lambda` travels towards the right horizontally with a velocity `V`. It strikes and reflects from a vertical plane surface , travelling at a speed `v` towards the left . The number of positive crests striking in a time interval of `3 s` on the wall isA. `3(V + v)//lambda`B. `3(V - v)//lambda`C. `(V + v)// 3 lambda`D. `(V - v)// 3 lambda`

A sound wave of wavelength ` lambda` travels towards the right horizon

1.	A sound wave of wavelength ` lambda` travels towards the right horizontally with a velocity `V`. It strikes and reflects from a vertical plane surface , travelling at a speed `v` towards the left . The number of positive crests striking in a time interval of `3 s` on the wall isA. `3(V + v)//lambda`B. `3(V - v)//lambda`C. `(V + v)// 3 lambda`D. `(V - v)// 3 lambda`
Answer» Correct Answer - A The relative velocity of sound waves w.r.t. the wall is `V + x`. Hence , the apparent frequency of the waves striking the surface of the wall is `(V + v)//lambda`. The number of positive crests striking per second is same frequency . In ` 3 s` , the number is ` [ 3 (V + v)]//lambda`.

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