Parts of a Circle

Anatomy of a Circle

Vocabulary

• Radius

• Diameter

• Circumference

• Pi

Radius

• a line segment extending from the center of a circle or sphere to the circumference

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Diameter

• a straight line passing through the center of a circle or sphere and meeting the circumference 

Circumferecne

• the outer boundary, especially of a circular area; perimeter

Pi

• By definition, pi is the ratio of the circumference of a circle to its diameter. In other words, pi equals the circumference divided by the diameter (π = c/d). 

A circle is a round shape. We see circular objects around us every day. When you get into a car, you are riding on four circular tires

 If you have any change in your pocket or purse, you have some circular coins.

Wherever we fi nd them, all circles have the same parts. Let’s look at the anatomy of a circle. Every circle has a center. 

The outside edge of the circle is made of points. All the points around the edge are the same distance from the circle’s center. 

Like other points in geometry, the center is identified with a single letter. Whatever letter names the center of the circle names the entire circle as well. 

Since the center of the circle below is point O, this is Circle O.

If you draw a line from one point on the outside edge of a circle, and go straight through the center point to another point on the outside edge of the circle, you have drawn the circle’s diameter. 

A radius of the circle is only half the distance of its diameter. It connects the center point to one point on the outside edge of the circle. 

The distance around the outside edge of a circle is called its circumference. It takes a little more than the measure of three diameters to go around a circle’s circumference. 

That is where we get the number Pi. Pi is a decimal number that begins with 3.14, and continues on forever. It can also be shown as the symbol.

Multiple Choice

Name two everyday objects mentioned by the passage that are circular.

• Coins and Cookies

• Tires and Coins

• Tires and Yo-Yo's

Multiple Choice

What is the relationship between a circle’s diameter and its radius?

• The radius is twice the size of the diameter

• The radius is ten times the size of the diameter

• The radius is half the size of the diameter

Multiple Choice

What is meant by a circle’s circumference?

• It is half of the circle

• It is the measurement all around the circle

• It is twice the size of the radius

Open Ended

In your own words, explain Pi.