What is rotation?

The idea

Rotation is the motion of an object in which all its parts move in circles around an imaginary line passing through it, called the axis of rotation.

Fans and tops

A ceiling fan turns. A spinning top turns. A merry-go-round turns.

What do all these have in common?

• All their parts go round a line
• All their parts move in straight lines
• All of them stay still

Turn four objects

Turn each object and follow the dots, which are its parts. Then show the axis.

The imaginary line

Every part moves in a circle. The circles go round a line through the object. We cannot touch this line, so it is called imaginary.

Rotation is the motion of an object in which all its parts move in circles around an imaginary line passing through it, called the axis of rotation.

Notes

Rotation is the motion of an object in which all its parts move in circles around an imaginary line passing through it, called the axis of rotation.

Check yourself

In rotation, the parts of an object move …

The axis of rotation is …

Which of these is rotating?

In a spinning top, which parts go round the axis?

• in circles — correct. Yes! The parts go in circles round the axis.
• only up and down. Rotation is turning round a line. It is not an up and down motion.
• in straight lines. In rotation the parts go round, in circles.
• the outer edge of the object. The edge is the path of the outer parts. The axis is the line they go round.
• an imaginary line passing through the object — correct. Yes! The parts go round this line.
• a real rod that must be inside every object. The axis is imaginary. There is no rod through the Earth.
• A ball carried in a straight line. Its parts move along straight paths. They do not go in circles round an axis.
• A pencil sliding straight along a table. All parts of the pencil move in straight lines, so it is not rotating.
• A ceiling fan that is switched on — correct. Yes! The blades go in circles round the axis.
• All the parts — correct. Yes! Every part goes in a circle, big or small.
• Only the parts at the very edge. Parts near the edge go in big circles, but the parts near the axis also go round, in small circles.
• None of the parts. When the top spins, every part goes round.