Name: ______________________________Grade 5 Class: _____ Date: _____________
Defining a Circle
A circle is a shape with all points the same distance from the center. A
circle is named by the center. The circle to the left is called circle A since the
center is point A. If you measure the distance around a circle (circumference)
and divide it by the distance across the circle through the center (diameter), you will
always come close to a same number, depending upon the accuracy of your
measurement. This number is approximately 3.14159265358979323846...(use 3.14) We
use the Greek letter
(pronounced Pi) to represent this value. The number
goes on
forever.
The
distance
around
a
circle
is
called
the
circumference.
The circumference of the circle is the perimeter of the circle. The distance
across a circle through the center is called the diameter.
The formula for circumference is C=
.d. Here C is stand for circumference and d is
diameter.
The radius of a circle is the distance from the center of a circle to any point
on the circle. If you place two radii (plural form of radius) end-to-end in a
circle, you would have the same length as one diameter. Thus, the diameter
of a circle is twice as long as the radius. This relationship is expressed in the
following formula:
, where
is the diameter and
is the radius.
Example 1: The radius of a circle is two centimeters. What is the
diameter?
Solution:
d =2xr
=2x2
= 4 cm
Example 2:
The diameter of a circle is three centimeters. What
is the circumference?
Solution:
C=
xd
= 3.14 x (3 cm)
= 9.42 cm
A chord is a straight line that joins two points on the circumference of a
circle.
A
A diameter is a chord passing through the center. It is the
C
longest chord.
B
Exercises
A) Label the parts of the circles indicated by the arrows.
a)
b)
●
c)
●
d)
B) Use a compass and a ruler to draw the following shape on a piece of scrap paper.
a) Set your compass for a radius of eight centimeters.
b) Put one end of the compass in the middle of your paper and draw a complete
circle.
c) Cut out your circle. What do you think about its dimensions?
d) Fold your circle in half.
e) Fold your circle in half again.
f) Open your circle. Where the lines cross in the middle is the center point. Label it
A.
g) From the center point to the outside of the circle is the radius. Label one of the
folds ‘radius’.
h) What is the length of the radius of your circle? __________________
i)
Label the diameter.
j)
What is the length of the diameter of your circle?________________