pointing in the radial direction of length one.

方向都从圆心指向该点模长为1
The advantage of introducing that guy is that if you like,

引入它的好处是如果你愿意的话
I can now write an equation for the acceleration as a vector.

我可以把加速度写成矢量形式
The magnitude is v square over r.

模长为 v^2 / r
The direction is -er.

方向是 -er
So er is a new entity I've introduced for convenience.

因此 er 是为了方便而引入的
It plays a big role in gravitation, in the Coulomb interaction.

它在引力和库仑力中都起着重要的作用
It's good to have a vector

所以引入这么一个模长为 1
pointing in the radial direction of length one.

方向沿圆心向外辐射的矢量作用很大
That's what it is.

就是这些了
That's really the heart of what I did last time.

这些就是上节课的主要内容
Then we did some projectile problems.

我们还研究了一些抛体运动