A small raft carrying a loudspeaker that emits a steady 400 Hz tone drifts downstream…
PHYSICS · Single correct · A Level · Physics · Waves · Waves
Problem
A small raft carrying a loudspeaker that emits a steady 400 Hz tone drifts downstream with the river current at 2 m/s. A swimmer floats 5 m behind the raft, carried by the same current at the same speed. Which of the following best explains why the swimmer hears the tone at its original frequency, while a person standing on the riverbank perceives a different pitch?
(a) Because water carries the sound, the Doppler effect depends only on the speed of the water relative to the ground, so observers floating with the current hear no shift.
(b) The source’s speed of 2ms−1 is so small compared with vsound that any shift is below the threshold of hearing for the swimmer.
(c) The swimmer and the raft share the same velocity, so there is no relative motion between them and no Doppler shift.
(d) The Doppler effect happens only when a source moves toward an observer; if source and observer travel in the same direction, no shift occurs.