In a way, a circle is like
a square. Why? Because it is as tall as it is wide. If you draw a
vertical line ( | ) across the
middle of a circle, it will measure the same as a horizontal line ( --
)going through the middle of the same circle.
What do we call the line
that goes from side to side through the middle of a circle? The answer is....
DIAMETER ( The stress is on the "A")!
If a circular shape is
longer in any direction, it is no longer a circle.
Open a new play-slide in
your Power Point presentation. We'll delete it later.
Select the BLANK slide
On your drawing toolbar,
select the circle. NOTE: To draw a perfect circle, hold down the SHIFT
key as you drag.
How can you check to see
if you drew a perfect circle? Fill the information in the sentence
(Click on the
review ratios and proportions in our last task. )
With the FORMAT OBJECT
panel open on the Size Tab, make the circle half as small.
The line around the circle
is called the CIRCUMFERENCE--long word for a big job!
How do we measure the circumference? Hmmmm. It's easy to measure the outside of a square or
rectangle. We just add the dimensions of each side. But a circle
doesn't have different sides to add.
In order to measure the
dimensions of a circle, you need to learn two more terms: radius and
of a circle in
the measurement from the middle of the circle to any of its sides. In
other words, the radius is half (1/2) of the
And what is PI? PI
represents the steady measurement of the distance around a circle
divided by the distance across its center. Don't worry much about
that. Just remember that when you want to find out how much the
outside of your circle (circumference) measure, you need to use the PI
value. That value is approximately 3.14. That's it. We use the Greek
(Pi) to represent this 3.14
Here are some formulas for
you to play with.
Let's call CIRCUMFERENCE
"C" so we don't have to write the big word every
Let's call RADIUS
Let's call PI, .
And let's call DIAMETER
Here we go.
ˇ D (The dot in the middle is the
same as times or X)
= 2 ˇ R
2 ( or D/2)
If a tire has a diameter
of 3 feet, what is it's radius? R
= D ÷
2 ( or D/2)
divided by 2 equals 1.5 feet!
It's as simple as that!
To practice these
formulas, click on the following sites and do their exercises. They
are a lot of fun.