Parts of a Circle

Pre-Algebra · Axiom Academy

From the center point to the number — the anatomy of a circle, drawn one part at a time. Every circle starts with a single center point. The radius is the distance from that center out to the edge — and the defining property of a circle is that this distance never changes . Watch one radius arm sweep all the way around: its tip stays exactly r from the center the whole time, and that traced path is the circle. Every point sits the same distance r from the center 2. Diameter: The Distance Across A diameter is a straight segment that runs through the center and touches the circle on both sides. The animation grows one radius, then a second equal radius the opposite way — laid end to end, the two equal radii ARE the diameter. So the diameter is always exactly twice the radius. If a circle has radius r ... — say , what is its diameter? Three more parts live on the same circle, and the easiest way to tell them apart is by shape . The animation draws all three on one circle: a straight chord, a curved arc, and a filled wedge. A straight segment joining any two points on the circle. The longest chord of all is the diameter. A curved piece of the circle's edge between two points — a slice of the boundary itself. A wedge bounded by two radii and the arc between them — a slice of pizza.

This is the written version of the interactive lesson above. See the full Pre-Algebra course.