Parts of a circle
Illustrate and name parts of circles, including radius, diameter, and circumference; know that the diameter is twice the radius
What a learner can do afterwards
- Label the radius, diameter, and circumference on a circle diagram
- Calculate the diameter given a radius of 4.5 cm (diameter = 9 cm)
- Explain the relationship between radius and diameter in their own words
The lesson
Every circle has a middle point called the center. The radius is the straight distance from the center to any point on the edge. No matter which direction you measure, the radius stays the same length.
The diameter is a straight line that crosses the whole circle, passing through the center and touching the edge on both sides. Since it covers two radius-lengths, the diameter is always twice the radius.
Maya measures a round plate and finds the radius is 4.5 cm. To find the diameter, she doubles it: 4.5 cm x 2 = 9 cm. The plate's diameter is 9 cm.
Circumference is the distance around the outside edge of a circle, like the edge of a clock face. Keep this rule handy: diameter = radius x 2.
Radius runs from the center to the edge, diameter crosses through the center at twice the radius, and circumference is the distance around the outside.
Watch it
Where it sits
Learn first
Nothing; this is a starting point.
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.