Four names for four parts of a circle
A circle holds four named parts. A radius joins the centre to a point of the circle.
- A circle, with its centre O.
- A chord — a straight line, both ends on the circle.
- An arc — a piece of the ring itself, between two points.
- A segment — shut in by one chord and one arc.
- A sector — shut in by two radii and one arc.
The four words
A chord is a straight line joining two points of the circle. A diameter is a chord that passes through the centre. It is the longest chord there is.
An arc is a piece of the circle itself. It has two ends. Two points cut the circle into two arcs. The shorter one is the minor arc. The longer one is the major arc. A diameter cuts the circle into two arcs of equal size.
A segment is a region, a flat area. One chord and one arc shut it in. Shut in means closed on every side. Each chord makes two segments, one on each side.
A sector is a region. Two radii and one arc shut it in. It looks like a slice cut from the centre outwards.
| Part | What it is | How to spot it |
|---|---|---|
| Chord | A straight line | Both ends on the circle. |
| Arc | A piece of curve | Part of the ring, between two points. |
| Segment | A region | One chord and one arc around it. |
| Sector | A region | Two radii and one arc around it. |
Try it. Name each coloured part.
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