when I want A and I want to know what'll it do next.

只要我知道 A 我就能知道接下来的情况
Remember, when we go back to the laws of Newton,

大家回忆一下当我们在讲牛顿定律的时候
the laws of Newton only tell you this --

它只告诉了我们这些
and we've been talking about this is useful information.

我们已经讲过这是有用的信息
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