Page 189 - Start Up Mathematics_5

Measuring Circumference of a Circle
Remember
Circumference of a circle is the length of its boundary.
Let’s learn to find the circumference of a given circle. Circumference is also called

Step 1: Draw a circle of any radius. Mark its centre and perimeter of a circle.
any point A on it.
Step 2: Take thread or wool. Keep its one end at point A.

Step 3: Take the thread or wool around the circle till you reach
point A again. A
O
Step 4: Cut it at this point.
Step 5: Open the cut thread or wool and measure it with a ruler.

Step 6: The length of the thread or wool you get is the circumference of the circle.

0 mm 1 cm 2 3 4 5 6 7 8 9 10 11 12 13 14 15

5
6
3
4
Relation between Diameter and Circumference 2 1 1/16''
Look at the table given below.

Radius (r) Diameter (d) Circumference (C) Circumference ÷ Diameter

2.5 cm 5 cm 15.7 cm 15.7 ÷ 5 = 3.14 (approx.)

5 cm 10 cm 31.4 cm 31.4 ÷ 10 = 3.14 (approx.)

7.5 cm 15 cm 47.1 cm 47.1 ÷ 15 = 3.14 (approx.)

We observe that circumference ÷ diameter = 3.14
Remember
or
The unit of circumference
Circumference = 3.14 × Diameter is same as the unit of
C diameter or radius.
C = 3.14 × d or d =
3.14

Example 2: Find the circumference of a circle whose diameter is 4 cm.

Solution: Diameter (d) = 4 cm
Circumference (C) = 3.14 × d
= 3.14 × 4 cm

= 12.56 cm

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