Circles and Amount of Turn

Lesson 2.1

I CAN…. I CAN use the amount of

turn to measure angles and use the relationships among parts of circles.

Vocabulary A circle is the set of points in a plane that

are the same distance from some point called the center.

The circumference of a circle is the distance around it.

Identifying the Parts of a Circle

Radius is a segment whose endpoints are the center of the circle and any points on the circle.

The length of such a segment is call the radius of the circle.

A diameter is a line segment with both endpoints on the circle that passes through the center.

A chord is a line segment whose endpoints are on the circle.

Chords do not have to pass through the center of the circle. However, if a chord does pass through the center, it is also considered a diameter.

Numerical Relationships A radius is exactly one-half of a diameter. Therefore a diameter is twice a radius.

5.5 cm

If the radius is 5.5 cm, then the diameter is ___________ cm.11

Circumference Remember, circumference is the distance

around the circle.

Exploring Further…What do you notice about the radii of the circle?

What do you notice about the diameter of the circle?

What do you notice about the radii compared to the diameter?

Homework Time