Question

A sound wave of wavelength λ 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 three seconds on the wall is

• A. 3 (V + v) /λ
• B. 3(V – v)/λ
• C. (V + v)/ 3λ
• D. (V – v) /3λ

Answer 1

Answer: (A)

3 (V + v) /λ

Answer 2

The relative velocity of sound waves with respect to the walls is V + v. Hence the apparent frequency of the waves striking the surface of the wall is (V + v)/ λ. The number of positive crests striking per second is the same as frequency. In three seconds the number is [3 (V + v)]/ λ.