During this part, when you're doing a quarter of a circle, this is supposed to be a quarter of a circle.

在这部分,当你沿着四分之一圆运动,这就是个一个圆的四分之一

耶鲁公开课 - 基础物理课程节选

So that a half circle, instead of calling it 180 degrees, And a quarter circle, instead of 90, It's good to know certain famous angles.

半圆,不说它是180°,四分之一圆,不叫90°,知道一些常用的角度很有必要

耶鲁公开课 - 基础物理课程节选