"Whenever there is an operation close to the border area where there is a possibility of either exchange of firing or crossing.

VOA: standard.2009.07.04

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