And you don't need to know too much more to do that.

你们已有的知识基本足够做这样的问题了
You're almost there.

你们差不多都会了
So, I'm going to just tell you the one missing thing.

只还差一件事现在我就要告诉你们
So, what Newton had done is to realize that

牛顿意识到
in order to understand motion of bodies,

要理解星体的运动
you have to associate a force with acceleration

就要把力和加速度联系起来
and not with mass.

而不是质量
That was his contribution.

这是他的贡献
If you think it's forces that cause velocity,

如果你们认为力产生了速度
you look at a planet going around the Sun,

看看绕着太阳运动的行星
where the velocity is pointing in different directions at different times

速度始终指向不同的方向
and you have to ask

你不得不问