The third law says that if there are two bodies,

第三定律说如果有两个物体
called one and two,

分别叫物体一和物体二
force of one on two is minus the force of the second on the first one.

一对二的作用力等于负的二对一的作用力
This is the thing about action and reaction.

这就是作用力与反作用力
All the laws that anybody knows have this property.

我们已知的任何定律都满足这一性质
What does it require to be a successful mechanic,

怎么才能当个成功的力学家
to do all the mechanics problems?

并能解决所有的力学问题呢
You got to be good at writing down the forces acting on a body.

你得擅长列出物体上的作用力
That's what it's all going to boil down to.

这就是这节课的最重要的要求
Here is my advice to you.

我有几条建议
Do not forget the existing forces

不要忘记已存在的力
and do not make up your own forces.

不要自己乱加力