Circumference of a Circle
⏱ 0:27📘 Grade 6 · Circles
Roll a circle and measure it with its diameter: the circumference is always about 3.14 times the diameter. Meet π and C = πd.
The videos are narrated in Mandarin Chinese with on-screen subtitles.
📚 What you will learn
- Diameter: the straight line through the centre from one side of the circle to the other. It is the longest line inside a circle, written d.
- Circumference: the distance all the way round the circle. Roll the circle one full turn and the distance it travels is its circumference, written C.
- Measure the circumference with the diameter: it fits 3 times and a little bit more.
- Pi (π): circumference ÷ diameter is always the same number, about 3.14.
- Formula: C = πd. Because the diameter is twice the radius, you can also write C = 2πr.
- Example: a diameter of 10 cm gives a circumference of 3.14 × 10 = 31.4 cm.
🎙️ What the video says (in Chinese)
- 小朋友们，我们来量一量，圆的周长！
- 这是一个圆，中间最长的线，叫直径。
- 滚一圈，走过的长度，就是周长。
- 用直径去量。一个！两个！三个！还多出一点点！
- 周长是直径的 3.14 倍，这个数叫圆周率，读作 π。
- 所以，圆的周长 = 圆周率 × 直径！
- 比如直径是 10 厘米，周长就是 31.4 厘米！
✏️ Try it yourself
A circle has a diameter of 4 cm. What is its circumference?
Show answer
3.14 × 4 = 12.56 cm
A round flower bed has a radius of 5 m. How long is the path around it?
Show answer
The diameter is 10 m, so 3.14 × 10 = 31.4 m
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