But there is no time limit on it.

但是没有时间限制。

麻省理工公开课 - 固态化学导论课程节选

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