The radius of a circle is 4cm, how many square centimeters is its area?

一个圆的半径是4厘米，它的面积是多少平方厘米？

youdao

That is a circle of radius 1. That's the unit circle.

是一个半径为1的圆，那是单位圆。

youdao

So, that's actually a great circle meridian of length, well, I mean, well, the radius is the radius of the sphere.

路程就是个大圆，我的意思是，这个圆的半径是球的半径。

youdao

That means, it can only be rattling around in a circle of radius R.

也就是说它只能,限制在一个半径为 R 的圆上运动

耶鲁公开课 - 基础物理课程节选

So we know it's moving on a circle of radius r.

因此这个质点始终在做半径为 r 的圆周运动

耶鲁公开课 - 基础物理课程节选