Grade 3Math
Circles and SpheresCenter, radius, and diameter
A round shape drawn with a compass is called a circle. Let's learn about the center, radius, and diameter of a circle, and about spheres, which are shaped like a ball.
The narration and the pictures are in Japanese. You can hear the same words that are used in class at a Japanese school.
- Video
- 1:42（9 scenes）
- Problems
- 2 problems
Let's try
Write the math sentence in your notebook first, then open the answer.
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1There is a circle with a radius of 4 cm. How many cm is the diameter?
Hint
How many radiuses long is the diameter?
Answer
It is 8 cm. The diameter is two radiuses long, so 4 + 4 = 8.
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2In one circle, is every radius the same length?
Hint
You drew it without changing the opening of the compass.
Answer
Yes, they are the same length. That is because a circle is a shape drawn so that the length from the center is always the same.
Words in this lesson
- circle 円
- A round shape drawn so that the length from one point is always the same.
- center 中心
- The point in the middle of a circle.
- radius 半径
- A straight line joining the center and a point on the edge of the circle.
- diameter 直径
- A straight line that goes through the center and joins two points on the edge of the circle.
- sphere 球
- A shape like a ball, which looks like a circle from any direction.
For parents and guardians
- The names of lessons and the order of topics differ slightly between textbooks. This page follows the content of Japan's national curriculum guidelines (the Course of Study).
- You can draw a circle by tracing the rim of a cup or the edge of a plate. To find the center, fold a paper circle in half, turn it, and fold it in half again; the point where the folds cross is the center.
- If you cut something close to a sphere, like a ball, the cut surface is close to a circle (real objects such as mandarin oranges are not exact spheres, so the cut surface is roughly a circle). Please also check together that the cut surface is biggest when you cut through the middle.