The way round and the way across
A bangle, a thali and a tank are all circles. The rim is the outer edge of a circle.
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- A circle, with its radius r and diameter d marked.
- Walk all the way round the rim. That length is the circumference.
- Straighten the rim into a line. It holds 3 full diameters.
- A small piece is left over. It is about 0.14 of a diameter.
The rule
Divide the circumference by the diameter. Every circle gives the same number.
Circumference — the distance all the way round a circle.
That number is called π (pi). Here we use 22/7 for π. It is close, not exact.
The radius r goes from the centre to the rim. The diameter d goes across, through the centre. The diameter is twice the radius, so d = 2r. Then circumference = π × d = 2 × π × r.
| Radius | Diameter | Circumference | Circumference ÷ diameter |
|---|---|---|---|
| 7 | 14 | 44 | 22/7 |
| 14 | 28 | 88 | 22/7 |
| 21 | 42 | 132 | 22/7 |
For radius 7: 2 × 22/7 × 7 = 44.
Work it backwards too. A circle has circumference 66 cm. Then 2 × 22/7 × r = 66. So r = 66 ÷ (2 × 22/7) = 66 × 7 ÷ 44 = 10.5 cm.
At home, wind a thin thread once round a bottle. Straighten the thread and measure its length. Measure the bottle's way across. Divide the thread's length by the way across. You get a number near 3.