The Doppler effect
3 marks · NormalA car is moving at a high speed with its horn sounding. Compare what an observer in front of the car hears to what an observer behind the car hears. Explain your answer.
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Common mistakes: Mixing up front/behind; Saying loudness changes only
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Marking points
B1 · 1 mark · States that in front, the observer hears higher frequency/pitch.
B2 · 1 mark · States that behind, hears lower frequency/pitch.
B3 · 1 mark · Explains as due to compression/stretching of waves from car's motion (Doppler effect).
Worked solution
The observer in front of the car hears a higher frequency (higher pitch) because the wavefronts are compressed as the car moves toward them.
The observer behind hears a lower frequency (lower pitch) as the wavefronts are spread apart.
This change in frequency on either side of a moving source is called the Doppler effect.