Objectives

• Identify ratio of circumference to diameter as pi.
• Find the circumference of circles given diameter or radius.
• Find diameter or radius of circles given circumference.
• Solve real world problems involving circumference of circles.

Vocabulary

Circumference
the measure of the distance around the outside edge of a circle.
Diameter
the measure of the distance across the center of a circle.
the measure of the distance half-way across the circle. It is the measure from the center to the outer edge. The radius is also half the length of the diameter.
Pi
the ratio of the diameter to the circumference, 3.14
Archimedes
a Greek mathematician and philosopher who identified 3.14 as pi.

Identify Ratio of Circumference to Diameter as Pi

To work with circles, we first need to review the parts of a circle. Let’s begin there.

We can measure several key parts of a circle. We can measure the distance across the center of the circle. This distance is called the diameter of the circle. Here is a picture of the diameter.

We can measure the distance from the center of the circle to the outer edge. This distance is called the radius. Notice that the radius is one-half of the measure of the diameter. Here is a picture of the radius.
We can measure the distance around the outer edge of the circle too. This distance is called the circumference of the circle. The circumference of the circle is the perimeter of the circle, only with circles we don’t call it perimeter we call the measure around the outside edge of the circle the circumference.

To understand things about circles, let’s consider some history.

Back in the time when the Greeks were discovering all sorts of things about mathematics, they were puzzled by mathematics and by the relationships between different measurements and geometry. The Greeks were famous for investigating ratios and proportions. When they studied different things, they knew that there was a connection between shapes and their measurements. Some of the Greeks thought a lot about circles.

Although the Babylonians had been investigating circles too, it was a Greek man named Archimedes who is credited with figuring out that there is a relationship between the distance across the circle or the diameter of the circle and the distance around the edge of the circle or the circumference of the circle.

Archimedes discovered that if you take the distance across the circle and stretch it around the circumference, that the length of the diameter will go around the circle 3 and a bit more times.

Let’s say that the diameter of this circle is 5 cm, well the circumference of the circle is three and a little more times the 5 cm.

Because of that, we say that the ratio of the diameter to the circumference is pi. We use the number 3.14 for pi because the ratio is a terminating decimal and does not end. However we use two decimal places for pi works for figuring out the circumference of the circle.

Here is the symbol for pi. When you see this symbol, you can use 3.14 in your arithmetic.