There, Newton doesn't tell you

在式子里牛顿没有告诉你
what forces are going to be acting on a body

在给定条件下有哪些力
in a given situation.

作用在物体上
If you leave the body alone,

如果你把物体丢在一边
maybe there's no force acting on it.

可能就没有外力作用在它上面
If you connect the body to a spring,

如果将一个弹簧连在这个物体上
which is neither compressed nor extended,

弹簧既没有被压缩也没有被拉伸
there's no force acting on it.

也不会有力作用在它上面
If you pull the spring,

如果拉这个弹簧
there is a force acting on it.

就有力作用在物体上了
Newton is not going to come and tell you

牛顿不会过来告诉你
what force the spring will exert when it's pulled by some amount.

弹簧被拉伸一定量时会产生多大的力