AS 91523 · 2024 paper · Doppler effect
Question 3(b) Merit
Practises Doppler effect
Jane and her classmates stand on a railway platform, listening to a train approach them at a constant speed, blowing its whistle. The speed of sound in air is 343 m s⁻¹.
| v sound | 343 m s⁻¹ |
|---|
Explain why Jane and her friends hear a change in frequency as the approaching train blows its whistle.
The 2024 paper, page 6 ↗ Marking schedule, page 5 ↗
See it move: The Doppler effect: a moving source and a moving observer
Work it out
Try it on paper first. Stuck? Take one hint at a time.
-
1 The train sends out one wavefront, then moves on a little before sending the next. Which way has it moved?
Towards the platform, following the wavefront it just sent. So the next wavefront starts nearer to that one than it would if the train were standing still. -
2 What does that do to the wavelength reaching the platform?
The wavefronts arrive closer together: the wavelength ahead of the train is shorter. -
3 Has the speed of sound changed?
No. The air sets it, 343 m s⁻¹, however fast the train goes. -
4 So what frequency do they hear?
By v = fλ, the same speed with a shorter wavelength gives a higher frequency: more wavefronts arrive each second, and the pitch is above the whistle’s own.
What the marker wanted
- Achieved One of: the wavefronts crowd together so the wavelength is shorter; or that sound still travels at the same speed. Bunching on its own, or the train’s speed being constant, does not count.
- Merit All three: the shorter wavelength, the unchanged speed of sound, and so a higher pitch.
In our words, from NZQA’s 2024 marking schedule.
✕Common mistake
Saying only that “the waves bunch up”, or that “the train’s speed is constant”. The schedule accepts neither on its own, and NZQA’s 2024 report lists bunching without the wavelength among answers that did not reach Achieved.How did it go?
Got it moves it further out. Review brings it back tomorrow.