155
Circle and disc
Textbook: Part 4, lesson 145, pp. 62–63
GoalCircle and disc; centre, radius, diameter; drawing a circle with a compass.
New words
circle (the line) · aylanadisc (circle with its inside) · doiraradius · radiusdiameter · diametr
Explanation
To draw a circle we need a compass: put the needle leg on the centre O and turn the pencil leg around it. The line we get is the circle, and the circle together with its inside is the disc. The segment from the centre to a point on the circle is the radius (OA). The segment through the centre joining two points of the circle is the diameter (BC). The diameter is 2 times as long as the radius.
Worked examples
If the diameter is 12 m, the radius is 12 : 2 = 6 m.
If the radius is 4 cm, the diameter is 4 × 2 = 8 cm.
Class activity
“Circle without a compass”: children draw a circle with string and pencil: a friend holds one end of the string at the centre while the pencil at the other end draws around. They say that the string’s length is the radius.
Practice
1
The circus arena has a diameter of 12 m. What is its radius in metres?
6
2
The radius of a circle is 4 cm. What is its diameter in centimetres?
8
3
A circus has 1000 seats. Children took half and adults took 350. How many seats are empty?
150
4
What is the segment from the centre to a point on the circle called?
radius
5
A show lasts 70 minutes: clowns 10, magicians 10, acrobats 7, singers 11 minutes. The rest is shared equally among 8 acts. How many minutes for each?
4