Mathematics
Intermediate
50 mins
Teacher/Student led
Learning Goals
Learning Outcomes
Lesson Files
+80 XP
What you need:
IWB/Projector/Large Screen

The Circle: Centre, Radius and Diameter

Learn the three special parts of a circle: the centre, radius and diameter. Discover why a diameter always passes through the centre and is twice as long as a radius.

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    1 - Getting Started ~4 mins

    Illustration for Getting StartedOn the board is a circle drawn by going round the edge of a cup, with one dot marked in the very middle.

    Look at the edge. Do you think every point on that round edge is the same distance away from the middle dot? Have a good look before you decide.

    👇 Mark your steps as done as you complete them.

    +5 XP

    2 - Watch and Notice ~9 mins

    Look at the circle on the board. The middle point is the centre.

    Key point

    A straight line reaches from the centre out to the edge. That is the radius. Every radius of the circle is exactly the same length, wherever it points.

    Key point

    A line goes straight across the circle and through the centre. That is the diameter. Notice how the diameter is made of two radii put together, one radius on each side of the centre, so it is twice as long as one radius. (Radii means more than one radius.)

    👇 Mark your steps as done as you complete them.

    +10 XP

    3 - Try It Together ~10 mins

    Now some of you come up to the board in turn to work with the circle, while the rest of the class watches and agrees or corrects out loud.

    Key point

    First one pupil points to the centre. Then another makes the radius bigger, then smaller, and we all watch that it always reaches from the centre out to the edge. Then another points to the diameter going straight across through the centre. Last, we all check together that the diameter is exactly two of those radii joined end to end.

    Explore the parts of the circle

    +15 XP

    4 - Draw and Label in Your Copy ~4 mins

    COPYBOOK MOMENT

    Here is a quick way to find the centre. Watch the scrap circle of paper being folded in half, then in half again. Where the two folds cross is the centre.

    Now in your maths copy, draw round a circular lid. Mark the centre with a dot in the middle. Then draw and label one radius and one diameter with your ruler.

    +10 XP

    5 - Class Challenge ~8 mins

    Now we work at the desks with real round things, sharing the objects around as you finish. Start with one round object and draw round it. Find and mark the centre with a dot. Draw one radius from the centre to the edge and label it. Draw one diameter straight across through the centre and label it. Then swap for another round object and do it again. Each time, say how you know your diameter passes through the centre.

    Hands-on Task

    +15 XP
    Pupil practice
    Module 7 · 2D and 3D Shapes and Angles Shape & Space
    Lesson 81 · The Circle: Centre, Radius and Diameter
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