The first thing you have to know in order to use the laws of Newton,

要使用牛顿定律首先必须知道
you have to separately know the left-hand side.

你必须单独知道等式左边的项
You have to know what force is going to act on a body.

必须知道哪种力作用在物体上
You cannot simply say,

不能只是简单地说
"Oh, I know the force on the body, it is m times a";

"我知道作用的物体上的力它等于 m 乘以 a"
m · a is not a force acting on a body;

m·a 不是作用在物体上的力
a is the response to a force;

a 是力产生的效应
you got to have some other means of finding the force.

你得用其它方法来找到作用力
And in this case,

在这个情况下
the force on the mass is due to the spring.

作用在物体上的力来自弹簧
So, I pull the spring by various amounts

所以我把弹簧拉伸不同长度
and I see what force it exerts.

并测量它所产生的力