An observer moves towards a stationary source of sound, with a velocity one-fifth of the velocity of sound. What is the percentage increase in the apparent frequency?

0.5%
zero
20%
5%

Correct answer: 20%

Solution

For an observer moving toward a stationary source, f' = f(v + v0)/v = f(1 + 1/5) = 1.2f. The apparent frequency rises by 20%.

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