Concentric circles
Concentric circles are two or more circles that share the same center point but have different radii, like the rings on an archery target or ripples in a pond. To draw concentric circles, keep the compass pivot fixed and simply change the radius for each new circle. In Grade 7 Saxon Math Course 2, Chapter 2, concentric circles are introduced as students build foundational geometry vocabulary. They reappear in coordinate geometry and real-world design contexts such as circular targets, cross-sections of cylinders, and ring-shaped areas.
Key Concepts
Property Concentric circles are two or more circles that share a common center point but have different radii.
Examples Draw a circle with a 3 cm radius, then, from the same center, draw another with a 5 cm radius. The rings on a target for archery are perfect concentric circles.
Explanation Think of a bull's eye target or ripples from a pebble in a pond! They all share the exact same center point but get bigger with different radii, creating a cool nested effect. You draw them by keeping your compass pivot point fixed and just changing its width.
Common Questions
What are concentric circles?
Concentric circles are circles that share the same center but have different radii. They look like the rings on a bullseye target.
How do you draw concentric circles with a compass?
Keep the compass needle at the exact same center point and draw circles by changing only the width (radius) of the compass for each circle.
What is a real-world example of concentric circles?
Archery targets, tree rings, ripples spreading from a rock dropped in water, and the cross-section of a pipe are all examples of concentric circles.
How do you find the area between two concentric circles?
Subtract the area of the inner circle from the area of the outer circle. This is called an annulus. For inner radius 3 and outer radius 5, area = π(5²) − π(3²) = 25π − 9π = 16π.
When do 7th graders learn about concentric circles?
Saxon Math, Course 2, Chapter 2 introduces concentric circles as part of the Grade 7 circle and geometry vocabulary unit.
Are concentric circles the same size?
No. Concentric circles share a center but have different radii, so they are different sizes. They never intersect each other.