First, I'm trying to convince you,

首先我在通过随机选取的例子
through an example that I selected randomly,

说服你们
that if you took a random force and

即如果我取任意一个力
found the work done along one path, or another path,

求解它沿着某条或另一条路径所做的功
you will in fact get two different answers, okay?

你实际上会得到两个不同的结果
That's the first thing, to appreciate that there's a problem.

这是第一个方面注意这里有个问题
So generally, if we took a random force,

一般来说如果我们取任意一个力
not a frictional force,

摩擦力除外
a force that depends only on location and not velocity,

这个力只取决于位置而非速度
it will not be possible to define a potential energy

这样就既不能定义出势能
nor will it be possible to define a K + U so that it doesn't change.

也不能定义出总值不变的 K + U
So, it's going to take a very special force

所以我们要选取一个特殊的力