You put a chunk of wood on a table and you try to move it at constant speed.

你把一大块木头放在桌子上,试着让它匀速运动

耶鲁公开课 - 基础物理课程节选

Whenever you see a particle moving in a circle, even if it's at a constant speed, it has an acceleration, v square over r directed towards the center.

只要看到质点做圆周运动,即使是匀速圆周运动,也存在一个加速度,大小为 v^2 / r,方向指向圆心

耶鲁公开课 - 基础物理课程节选