The velocity of the center of mass has not changed as seen from your frame of reference, because the velocity of center of mass is always the same, remember, because momentum is conserved.

质量重心的速度，并没有发生改变，你在参照系中可以看出，因为质量重心的速度，总是保持不变，要记住，由于动量守恒。

youdao

I will then transfer to the center-of-mass frame, and I will show you, then, this quite amazing property.

我将它转化到重心体系，我会给你们展示，这是神奇的性质。

youdao

So to get into the center-of-mass frame, you must subtract the velocity of the center of mass from the initial velocities.

要进入重心体系，你就必须减去速度，重心的速度，从原始速度中减去。

youdao