R One is the position vector, r.

一边是位矢。

And, we look at how the position vector changes.

来看看位置矢量的改变。

So, it's the second derivative of a position vector.

它是位置向量的二阶导。

Well, I claim that the position vector is in the plane.

我断言，这个位置矢量在平面上。

And this, now, is the position vector of the center of mass.

现在，这里是，物体的矢量中心。