Now, how do we get this unit vector?

那我们怎么得到这单位矢量呢？

It is not a unit vector.

它不是单位向量。

It's the unit vector in the vertical direction.

即竖直方向的单位向量。

So, that means this actually should be the unit vector.

这意味着这一项就是单位向量。

Now, another useful notion is the unit vector to the trajectory.

还有一个有用的概念就是轨道的单位矢量。