# PowerPoint Presentation - Keming Primary School

Document Sample

```					                        M6.1b

Circles
Primary 6 Mathematics
Circles

Chapter Learning Outcomes

 Recognise   semi-circles &
Use  the formula to find
circumference of circle

2
Circles

Parts Of A Circle - Circumference
 Theperimeter of a circle is called the
circumference.

 To measure the
circumference of a circle or
circular object:
   Wrap a length of string around it and
use your finger or marker to indicate the
end
   Place the string flat against a ruler to

3
Circles

Parts Of A Circle – Circumference
Activity 2
     Draw the circles with the given measurements,
then complete the table.
     What do you notice about the values in the
last column?
Circle    Diameter   Circumference   Circumference  Diameter
(correct to 2 decimal places)
A         2 cm
B        3.5 cm
C         5 cm

4
Circles

Parts Of A Circle - Circumference
Circle   Diameter   Circumference   Circumference  Diameter
(correct to 2 decimal places)
A       2 cm        ≈ 6.3 cm              ≈ 3.15
B       3.5 cm      ≈ 11 cm               ≈ 3.14
C       5 cm        ≈ 15.7 cm             ≈ 3.14

   What do you notice about the values in the
last column?
   The circumference of any circle when divided
by its diameter always has the same value.
5
Circles

Parts Of A Circle - π
   We usually take the value of π to be 3.14
or 22 .
7
   You can also use the value of π in your
calculator.

 Therefore:

Circumference = π  Diameter
Circumference = 2  π  Radius
6
Circles

Example
The diameter of a wall clock is 15 cm.
Find its circumference. (Take π = 3.14)

Circumference = π  Diameter
= 3.14  15 cm
= 47.1 cm

7
Circles

Example
The radius of a circular coaster is 7 cm.
22
Find its circumference. (Take π= 7 )

Circumference = 2  π  Radius
22
= 2  7  7 cm
= 44 cm

8
Circles

Example
A wheel has a diameter of 60 cm.
Find its circumference.
2 decimal places.

Circumference = π  Diameter
= π  60 cm
≈ 188.496 cm
≈ 188.50 cm
9
Circles

Semi-circles & Circumference
   A semi-circle is half a
circle.

    Circumference of a         1
2
semi-circle
1
= ( 2  Circumference of
circle) + Diameter

10
Circles

quarter of a circle.

   Circumference of a         1
4
1
= ( 4  Circumference of