Learning about Circles

Diameter and Radius

• The diameter is the distance across a circle through the center. You can see what the diameter looks like by drawing a line through a circle, making sure it goes through the point in the middle, called the center.
• The radius is the distance from the center to a point on the circle. The radius is exactly half the length of the diameter.

 

 

Diameter and Radius Mini-Movie Click the button on the left to draw the diameter. Click button on the right to draw the radius. After it is drawn, it will move to show that it is half the length of the diameter.

Use this mini-movie on your SMARTBoard

Circumference

Circumference is the perimeter of a circle, or the distance around the outside the circle.

Circumference Mini-Movie Click the button to make the red dot trace the circumference of the circle. The radius and diameter are also shown so that you do not confuse them with the circumference.

Use this mini-movie on your SMARTBoard

Formula for finding the circumference of a circle The circumference of a circle is approximately 3.14 times the size of its diameter. We know the number 3.14 as π, or pi. So the formula for finding the circumference of a circle is: C = dπ where d = diameter and C = circumference   Since the diameter of a circle is twice as long as the radius, we can also use another formula: C = 2πr where r = radius and C = circumference

 

Tangent

A tangent is a straight line that just touches a circle. In fact, it only touches the circle at one point. The tangent line is perpendicular to the radius drawn to this point.

Tangent Mini-Movie Click the button on the left to see on line meet the circle at a point and form a tangent. Click the button on the right to see another one.

Use this mini-movie on your SMARTBoard

 

 

 

Everything Geometry

The Basics| Classifying Quadrilaterals | Classifying Triangles

Area & Perimeter | Angles | Circles