Yes, because that's the Third Law of Newton.

没错因为要满足牛顿第三定律
Whatever the underlying force,

不管其中隐含着什么样的力
gravity, spring, anything,

重力弹簧的弹力或者别的什么
force on 1 due to 2, and force on 2 due to 1 will cancel,

2 对 1 的作用力和 1 对 2 的作用力会抵消
and everything I get today,

今天讲的所有东西
the whole lecture is mainly about this one simple result,

整节课主要就是关于这个简单的结论
this cancellation.

这两项抵消了
Then, this whole thing, I'm going to write as Fe,

然后整个这一部分我记为 Fe
meaning the total external force on this two-body system.

就是这个二体系统受到的合外力
So, I have this peculiar equation.

这样我就得到了这样一个奇怪的等式
I'm going to rewrite it in a way

我将会把它写成另外一种形式
that brings it to the form in which it's most useful.

让它变成一种最有用的形式