A perfect circle at a steady speed

The idea

Uniform circular motion is motion along a circular path at constant speed, the same at every point; it is an idealised model, as real motions often lack exactly constant speed and a circular path.

Round and round

A toy train goes round a circular track. We say it moves at a steady speed all the time.

Which two things must stay the same for the motion to be called uniform circular motion?

• The shape of the path (a circle) and the speed
• The colour and the size of the train
• The mass of the train and the length of the track

Four points

Press Next step to see uniform circular motion and then a real motion.

A useful model

When the path was a circle and the speed was constant, the arrows were all equally long. Real motions seldom match this exactly, but the model is simple, and it is a good starting point for harder real cases.

Uniform circular motion is motion along a circular path at constant speed, the same at every point. It is an idealised model, as real motions often lack exactly constant speed and a circular path.

Notes

Uniform circular motion: circular path and constant speed (same at every point). It is an idealised model of real motions.

Check yourself

Uniform circular motion is motion…

In uniform circular motion, the speed of the object is…

Why is uniform circular motion called an idealised model?

Which pair of conditions defines uniform circular motion?

• along a circular path at constant speed — correct. Yes!
• along a straight line at constant speed. That is uniform motion in a straight line. Uniform circular motion has a circular path.
• along a circular path with a speed that keeps changing. In uniform circular motion the speed is constant.
• greater at some points than at others. In uniform circular motion the speed is constant, so it is the same at every point.
• the same at every point of the circle — correct. Yes!
• zero at the starting point. The object is moving, so its speed is not zero. It is the same at every point.
• Because real motions have exactly constant speed and a perfectly circular path. It is the other way round: real motions often lack exactly constant speed and a circular path.
• Because objects moving in a circle have no speed. They do have a speed. In uniform circular motion it is constant.
• Real motions often do not have exactly constant speed and an exactly circular path — correct. Yes!
• Constant speed and a circular path — correct. Yes!
• Constant velocity and a circular path. On a circular path the direction of the velocity keeps changing, so the velocity cannot be constant.
• Constant acceleration and a straight path. A straight path is not circular, so this is not circular motion.