by Ms. B. L. Norton

This lesson was created as a part of the SMART website and is hosted by
the Illinois Institute of Technology

The purpose of this lesson is to introduce students to calculations associated with circles and applying those calculations to various sized circles.

A circle is a shape in which all its points are the same distance for the center.

Can you think of some objects that are shaped like circles?

Before we start, let's take a look at some "circular" terms.

The point located in the center of the circle is called the **origin**.
Notice that the circle has a letter in the inside next to
the center. This letter names the circle. In my example
given, the
circle will be called "circle B".

If I picked a point on the circle and drew a line from that point to
the
center of the circle, that would create a line of **radius (r)**.
Similarly, If I drew a line from the center of the circle to another
point on
the circle, that would also create a radius.

Notice
that I now have two (2) **radii **( pronounced ray-dee-eye), which
is the plural form of the word radius.

I have just created another part of a circle called the **diameter
(d)**.
The diameter is a straight line which originates at a point on the
circle,
passes through the center point or origin of the circle, and ends at a
point on the opposite end of the circle.

The distance around a circle is called the **circumference (C)**.

There is a relationship that exists between the circumference and
the
diameter. If you know the measurement of the circumference and
divide it
by the diameter, there is a constant number derived that we call __ Pi__.

This is the symbol for Pi...

Pi is equivalent to 3.1459265898....

Let's use 3.14 or 22/7 to represent Pi. (that's easier to remember and work with.)

What does the Energizer bunny and the number Pi have in common? (Hint: click here )