The Doppler Effect
3 marks · NormalA graph below shows how the frequency of a sound heard by an observer changes as a vehicle passes by:
Describe the main feature shown on the graph.
Explain, using the Doppler effect, why the frequency drops suddenly at around t = 5 s.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) The frequency increases, peaks, then suddenly drops at around t = 5 s.
(b) Before t = 5 s, the vehicle is approaching, so frequency is higher; at t = 5 s it passes the observer, after which it moves away so frequency drops due to the Doppler effect.
Common mistakes: Not linking the frequency drop to the change in the vehicle's motion relative to the observer.; Incorrectly describing frequency before and after vehicle passes.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Describes that the frequency peaks then drops suddenly at a given time.
M1 (b) · 1 mark · Relates high frequency before t = 5 s to vehicle approaching due to Doppler effect.
A1 (b) · 1 mark · Explains frequency drops after t = 5 s because vehicle is moving away, leading to lower frequency.
Worked solution
(a) The frequency increases, reaches a maximum, and then drops suddenly at around t = 5 s.
(b) As the vehicle approaches the observer, sound waves are compressed, so the observer hears a higher frequency (Doppler effect). When the vehicle passes the observer (t = 5 s), it moves away, causing the waves to be stretched out, so the observer hears a lower frequency.