The memorial has a large wall shaped in a half-circle.

VOA: special.2009.05.25

Now take these two fingers-- the middle finger and the thumb-- and as much as possible, from that 90 degree angle, create a circle.

再让这两个手指-,中指和姆指-,尽可能保持直角时围成一个圆圈。

哈佛公开课 - 幸福课课程节选

We know it's going around in a circle because if I find the length of this vector, which is the x-square part, plus the y-square part, I just get r square at all times, because sine square plus cosine square is one.

我们之所以知道它做圆周运动,是因为我求出了这个矢量的模长,也就是 x 的平方加上 y 的平方,我就得到了它在任意时刻的模长平方,因为正弦平方加余弦平方始终等于1

耶鲁公开课 - 基础物理课程节选