AS Daily A Level Physics question
In a ripple tank, a motor-driven dipper produces plane water waves that pass a fixed point P. The motor speed (frequency) and water depth are unchanged, but the dipper’s oscillation amplitude is reduced by 40% using the controller. Assuming linear waves and negligible reflections, what happens to the average wave intensity measured at P?
Answer
The correct answer is A.
Correct: A — 36% of its original value. A is correct because reducing amplitude to 0.60 of its original makes intensity scale as (0.60)² = 0.36 of the original. B is incorrect because it assumes intensity is directly proportional to amplitude, not the square of amplitude. C is incorrect because it confuses a 40% reduction in amplitude with the new intensity, ignoring the square relationship. D is incorrect because 64% would correspond to a 36% decrease, not the required square-law change from a 40% amplitude reduction.