So, everything else is called "external."

因此任何研究对象以外物体都叫"外部"
And I have 1 and 2, for example, are connected by a spring.

例如我现在让 1 和 2 之间连一根弹簧
The spring is not that important.

弹簧本身不是那么重要
It's a way of transmitting force from one body to the other.

只不过是用来把力从一个物体传向另一个物体
If you compress the spring and let it go,

如果你压缩弹簧然后放手
these two masses will vibrate back and forth

这两个物体就会在对方的作用力
under the influence of the other person's force.

影响下来回振动
That's an example of F12 and F21.

这就是 F12 和 F21 的例子
For example, if the spring is compressed at this instant,

如果弹簧在这一刻被压缩
it's trying to push them out;

它会努力把两个物体向外推
that means, really, 1 is trying to push 2 out towards that way,

也就是 1 想把 2 向这个方向推
2 is trying to push 1 to the left, this way.

而 2 则想把 1 推到左边这个方向