Right now at this point in our course, whenever you have a problem where there is some body and someone says, "Write all the forces on it", what you have to do is very simple.

现在在我们这个课程里,如果你遇到一道题目,题目给了你一个物体,并要求你,"写出该物体所受的所有力",你要做的其实很简单

耶鲁公开课 - 基础物理课程节选

So, the point is whenever you have derivatives with something over dt, do not hesitate to cancel the dts This will be actually true as long as both quantities are vanishingly small.

关键在于,只要你推出的式子分母上有dt,不要犹豫,直接约掉,这个式子只在,两遍都是无穷小的情况下成立

耶鲁公开课 - 基础物理课程节选