Meet the parts of a circle, wrap a string around it, slice it up to find its area. Play, solve missions, crack puzzles and beat the clock.
Start here. A circle is every point that is the same distance from one special point in the middle, called the centre. A line from the centre to the edge is a radius. A line across the circle that goes through the centre is a diameter. The diameter is always twice the radius, because it is two radii placed end to end. The distance around the edge is the circumference.
The magic number. Take a string and wrap it once around a circle. Now lay it straight next to the diameter. It is a little more than three diameters long. Every circle in the world does this, big or small. That "three and a bit" is called pi (π), and in this game we use π = 3.14. So the circumference is 3.14 × diameter. Using the radius it is 2 × 3.14 × radius.
Area of a circle. Cut a circle into thin slices like a pizza and lay them side by side, one pointing up and the next pointing down. The more slices you cut, the more the shape looks like a rectangle. Its height is the radius. Its width is half the circumference, which is 3.14 × radius. So the area is 3.14 × radius × radius. Area is measured in square units such as cm².
Common mix-ups. The radius is only half the diameter, so check which one the question gives you. The circumference is a length, but the area is a count of squares, so they have different units. When you double the radius, the circumference doubles, but the area becomes four times bigger.
Try this above. In Play, change the radius and watch the diameter. Then wrap the string around a circle with diameter 5. Last, slice a circle into 4, 8, 16 and 32 pieces and see the rectangle appear.
The radius goes from the centre to the edge. The diameter goes from one side of the circle to the other through the centre. The diameter is twice as long as the radius.
Pi (π) is the number you get when you divide the circumference of any circle by its diameter. It is a little more than 3. Its digits never end, so we use 3.14 as a good approximation.
Multiply the diameter by 3.14. If you know the radius, double it first. A circle with a diameter of 10 cm has a circumference of 3.14 × 10 = 31.4 cm.
Multiply 3.14 × radius × radius. A circle with a radius of 5 cm has an area of 3.14 × 5 × 5 = 78.5 cm². If you are given the diameter, halve it first to get the radius.
Just starting: the parts of a circle, radius and diameter. I know the basics: circumference and the 3.14 idea. Challenge me: area, half circles, rings and two-step problems.
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