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

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

The string bed in a longer frame generates more spin due to increased velocity.

因为较长的球拍可以产生较快拍头速度，所以给球的旋转也越强。

youdao

So the Earth is an inertial frame of reference, if you go in a train relative to the Earth at constant velocity, you're also inertial.

所以,地球是一个惯性参考系,如果你坐在相对于地球匀速运动的火车里,你也处于惯性状态

耶鲁公开课 - 基础物理课程节选

When you have a velocity in the moving plane frame and you want to find the velocity on the ground, you should add to every object in the plane the velocity of the plane with respect to the ground.

当你在运动的飞机上运动,然后你想找到相对于地面的速度,你就应该给每个物体加上,飞机相对于地面的速度

耶鲁公开课 - 基础物理课程节选