The Doppler effect
4 marks · NormalA student observes a train moving towards a platform while sounding its horn.
Describe what happens to the frequency of the sound heard by a person standing on the platform as the train approaches.
Explain your answer to part (a).
What will happen to the frequency of the sound as the train passes and moves away from the platform?
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.
(b) The train moves towards the observer so the sound waves are compressed, resulting in a shorter wavelength and higher frequency heard due to the Doppler effect.
(c) The frequency decreases.
Common mistakes: Pitch instead of frequency; Mixing up direction
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States frequency increases as train approaches.
B2 (b) · 1 mark · Explains waves are compressed in front of source.
B3 (b) · 1 mark · Links to higher frequency (or shorter wavelength) heard (Doppler effect).
B4 (c) · 1 mark · States frequency decreases as train moves away.
Worked solution
a) The frequency increases.
b) As the train moves towards the person, the sound waves are compressed, so the wavelength is shorter and the passenger hears a higher frequency. This is due to the Doppler effect.
c) Once the train passes and moves away, the frequency decreases.