Car Passing With Sound Waves

A car moves to the right along a road, sending out repeating wavefronts. Because the car moves, the wavefronts bunch closer ahead and spread farther apart behind, so a person at the roadside hears a higher pitch as it approaches and a lower pitch after it passes.

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Every lesson is open to everyone for 7 days after it is published. This one came out on September 14, 2026: its text stays readable here, and its picture opens with a subscription.

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Age 18: open this week

In the picture

Badge 1 sits on the car.
Badge 2 sits on the compressed wavefronts ahead of the car.
Badge 3 sits on the stretched wavefronts behind the car.
Badge 4 sits on the pedestrian at the roadside.

Examples

An ambulance siren sounds higher as it approaches and lower after it passes.
A race car engine note drops as it speeds past.

Key takeaways

Motion between a source and an observer changes the observed frequency.
Approaching motion raises pitch, while receding motion lowers pitch.

Carry on in the app

Three questions on this lesson. It has left the free week, so its quiz is for subscribers.

At least one new illustrated lesson every weekday for your child’s year, with its quiz and a reminder at 6:15 pm.

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