← Describing rotation

Spotting circular motion

Astrid rides a carousel horse. The horse carries her around the ride's center pole, all the way around, again and again.

A carousel horse, a car on a test track, a kite in the wind: plenty of paths curve. Which ones count as circles?

Measure from the center pole to Astrid's horse: the distance is 3 m.

A quarter of the way around, the distance is still 3 m.

pole3 m3 mthe horse, at the starta quarter of the way aroundboth bars read 3 m
The horse's path keeps one distance from the pole: circular motion.

All the way around, every point of the horse's path sits exactly 3 m from the pole.

This is circular motion: motion along a path that stays at a fixed distance from a central point.

A horse on the outer row rides a bigger circle, 5 m from the pole. Its path keeps that one distance too, so its motion is also circular.

Now a car on a circular test track: every point of its lap sits 50 m from the track's center. Circular motion again.

center50 mcarevery point of the lap sits 50 m from the center
Every point of the lap sits 50 m from the center: circular motion.

The same car then turns off onto a winding road. The road curves the whole way, but no single point stays at a fixed distance from the car. That motion is not circular.

winding roadone trial center point40 m65 m90 mthe car, at three momentsno single point keeps one distance to the car
The road curves the whole way, but no single point stays at a fixed distance: not circular.

A path can curve from start to finish and still not be a circle. The test is one question: does the path stay at a fixed distance from a central point?

The tip of a fan blade spins around the fan's hub, staying 0.30 m from it. Is the tip's motion circular?

A skier follows a winding slalom course down a hill. Is the skier's motion circular?

The tip of a wall clock's second hand sweeps around the clock's center pin, always 10 cm from it. Is the tip's motion circular?

pin10 cmthe second hand's tip
The second hand's tip stays 10 cm from the center pin.

The tip stays at one fixed distance, 10 cm, from one central point, the pin.

Yes: the tip's motion is circular.

A hawk glides in a wide loop that keeps tightening toward its nest. Is the hawk's motion circular?

nestearlierlaterhawkthe hawk's distance from the nest keeps shrinking
The hawk's loop keeps tightening toward the nest.

The loop curves the whole way.

But the hawk's distance from the nest keeps shrinking, so no single point stays at a fixed distance.

No: the hawk's motion is not circular.

Bruno says: 'A go-kart driving around a circular track is in circular motion, because it stays at a fixed distance from the track's center.' Is Bruno correct?

Your turn

Which motion is circular?

A ride designer checks whether a cart's path is circular. Which feature of the path decides it?

A camera drone flies one lap of an oval course and returns to its start. Its distance from the course's center changes from 20 m out to 35 m as it goes. Which feature of the lap shows the motion is not circular?