It wasn't the fact that they were running that time and they say "Oops! We are over the speed limit!

不是因为跑到那个时间,就说“不好，超过速度极限了！

哈佛公开课 - 幸福课课程节选

That limit will be some arrow we can call the velocity at the time and it will always be tangent to the curve.

那个极限也就是一个矢量,我们称之为瞬时速度,并且它总是和轨迹相切的

耶鲁公开课 - 基础物理课程节选

In that limit, we can measure velocity right now.

取极限的话,我们可以马上测出速度

耶鲁公开课 - 基础物理课程节选

So v of t, v is the limit, and we use the symbol dx/dt for velocity.

点的速度等于一个极限,我们用符号dx/dt表示速度

耶鲁公开课 - 基础物理课程节选