What a circle really is
Pin one end of a string. Tie a chalk to the other end. Pull the string tight. Move the chalk all the way round.
Loading lab…
The rule
Step 1 of 4
- A pin holds a string at the centre.
- Pull it tight. The string's length is the radius.
- Walk the chalk all the way round.
- Every point is one radius out. That is the circle.
The curve is the line the chalk draws.
| Word | What it means |
|---|---|
| Centre | The fixed point everything is measured from. |
| Radius | The fixed distance from the centre to the curve. |
| Circle | Every point at that distance from the centre. |
A compass does the same job. Its span, the gap between its legs, is set to the radius.
Example
The radius is 30 mm. The mm means millimetres. A point 30 mm from the centre is on the circle.
A point 22 mm out is inside. 22 mm is less than the radius.
A point 35 mm out is outside. 35 mm is more than the radius.