When you're driving in your car, there's a needle and the needle says 60; that's your velocity at this instant.

当你在开车时,有一根指针指着60,表示你这个时刻的速度

耶鲁公开课 - 基础物理课程节选

At this instant, the planet may be moving at that speed.

在这个时刻,行星可能在以这个速度运动

耶鲁公开课 - 基础物理课程节选

We can think of this as the decision happening at the instant at which you kicked.

我们可以认为罚球队员,在球被踢出的瞬间才做出决定

耶鲁公开课 - 博弈论课程节选

That is the whole triumph of calculus is to know that by looking at the position now, the position slightly later and taking the ratio and bringing later as close as possible to right now, we define a quantity that we can say is the velocity at this instant.

这就得靠微积分了,通过了解它现在的位置,和一小段时间以后的位置,计算它们的比,再让时间间隔尽可能缩短,我们就定义了一个,被我们称之为瞬时速度的物理量

耶鲁公开课 - 基础物理课程节选