Properties of Waves
4 marks · NormalA car horn is sounding as the car approaches and then passes you while you stand at the roadside.
Describe what happens to the pitch of the sound you hear as the car approaches, passes you, and moves away. Explain your answer using wave ideas.
What is the name of this effect, and does it happen for all types of waves?
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(a) Pitch is higher as the car approaches (waves are closer, frequency increases), drops to lower as it moves away (waves are more spread out, frequency decreases). This is because the motion of the source changes the wavelength and frequency of sound reaching you. · must show frequency increases when approaching, decreases when moving away, connect to pitch, wave explanation
(b) Doppler effect; it happens for all types of waves. · must show Doppler effect name, all waves
Common mistakes: Saying sound is louder rather than higher frequency; Omitting explanation for both approaching and receding
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Marking points
B1 (a) · 1 mark · States pitch is higher as car approaches, lower as it moves away.
B2 (a) · 1 mark · Correctly explains that this is due to the changes in frequency caused by the motion of the source.
B3 (a) · 1 mark · Makes clear use of wave terms (wavelength, frequency, wavefronts).
B4 (b) · 1 mark · Names the Doppler effect and states it happens to all types of waves.
Worked solution
a) As the car approaches, the sound waves are compressed, so frequency increases and pitch is higher. As it moves away, the waves are stretched, frequency decreases and pitch is lower. This is due to the change in distance between wavefronts reaching the observer.
b) This is called the Doppler effect and it occurs for all waves, not just sound.