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

The velocity is with respect to a fixed frame of reference.

此速度是相对于固定标架的。

youdao

Mechanics Kinematics: trajectories, velocity, acceleration, motion of rigid bodies, change of reference frame.

力学运动：轨迹、速度、加速度、运动、刚体动力学变化的参考帧。

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