Circles

Hello, and welcome to Myriora Maths! A circle is perfectly round. Every point on its edge is the same distance from the middle. Let's learn the names of its parts.

The middle point is called the centre. The edge all the way round is called the circumference.

A straight line from the centre to the edge is a radius. Every radius of a circle is the same length.

A straight line right across the circle, through the centre, is a diameter. It's made of two radii, end to end. So the diameter is double the radius.

To draw a circle, use a pair of compasses. Open them to the radius you want, put the point on the centre, and turn. The pencil stays the same distance from the centre all the way round.

Let's try one together. A circle has a radius of four centimetres. What is its diameter? Say it out loud!

The diameter is double the radius. Four times two is eight centimetres!

Here's a question that catches lots of people. A circle has a diameter of ten centimetres. What is the radius? Some people say twenty, doubling it. But the diameter is the long one, right across. The radius is half of it: five centimetres.

Your turn! A wheel has a diameter of sixty centimetres. What is its radius? Pick the right card. You've got twenty seconds, or pause the video. Go!

Time's up! The radius is half the diameter. Half of sixty is thirty centimetres!

Quick review. The centre is the middle of a circle. The radius goes from the centre to the edge. The diameter goes right across, through the centre, and it's double the radius. And the edge is called the circumference. See you next time!