Parts of a circle

In your book, Maths Mela (Chapter 1), you mark a point A and then draw many points that are all the same distance from A. Joining them gives a circle. The point A is the centre of the circle. A line from the centre to the border of the circle is called a radius.

Now take a paper circle and fold it in half. Open it and measure the crease. Fold it in half a different way and measure again, then a third way. All the creases are the same length! Each crease goes right across the circle and passes through the centre. A line like that is called a diameter. All the diameters of a circle are equal.

Fold the circle in half, and then in half again. You can see that the diameter is made of two radii, one on each side of the centre. So the diameter is double the radius, and the radius is half the diameter. If the radius is 5 cm, the diameter is 5 + 5 = 10 cm.

All wheels look like circles, but some are bigger than others. A bigger wheel has a longer radius and a longer diameter. If one wheel has a radius of 30 cm and another has a radius of 20 cm, the first wheel is bigger and its diameter (60 cm) is longer than the other's (40 cm).