AS 91523 · 2024 paper · Doppler effect

Question 3(a) Achieved

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

Describe what Jane and her friends hear, compared with the driver, 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 Is the whistle moving towards or away from the driver?
Neither. The driver travels with the train, so the driver hears the whistle’s actual frequency.
2 And relative to Jane and her friends?
It is coming towards them, and a source approaching a listener is heard at a higher frequency.

Answer A higher frequency, so a higher pitch, than the driver hears.

What the marker wanted

Achieved A higher frequency.

In our words, from NZQA’s 2024 marking schedule.

Common mistake

Saying the whistle sounds louder. NZQA’s 2024 report lists mixing up volume and frequency among answers that did not reach Achieved: the question is about pitch.

How did it go?

Got it moves it further out. Review brings it back tomorrow.