You have to have a good public transportation system when you have as many people here as we do.

如果你那儿和我们这儿的人一样多，你们得有一个良好的公共交通系统。

纽约的交通手段 - SpeakingMax英语口语达人

If you want to know how fast it's moving at a given time, if you want to know the velocity, I just take the derivative of this answer, which is 10-10t.

如果你想知道它在给定时刻的运动有多快,如果你想知道它的速度,我只要对这个式子求导,得到10-10t

耶鲁公开课 - 基础物理课程节选