When you learn relativity,

当你们学习相对论时
you will find you have one observer saying something,

你会发现某个观察者看到这样
another observer saying something.

另一个观察者看到那样
They will disagree on a lot of things,

他们的很多看法都会不同
but there are few things they will all agree on.

但仍有些东西是可以达成共识的
Those few things will be analog of A·B.

就是类似于 A·B 的东西
So, it's very good to have this part of it very clear in your head.

所以你最好清楚明白这个概念
This part of elementary vector analysis

初级矢量分析的这一部分
should be clear in your head.

你们应该清楚
Okay, so any questions about this?

好了还有什么问题吗
So, if you want, geometrically,

从几何学方面来
the work done by a force

表示出力所做的功