As to the conservation of motion of center of mass, the method of absolute coordinate system is generally used in the teaching meterials of theoretic mechanics.

对于质心运动守恒问题，理论力学教材中一般用绝对坐标法进行计算。

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And ultraviolet light will interact with that little ball, the center of mass, and when I then make it dark, you will only see the motion of the center of mass.

这紫外线光可以，与小球互动，也就是质量重心，当周围暗下来，能看到质量重心运动。

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So I mentioned to you that the motion of the center of mass is very uniform in the absence of external forces, and that I can demonstrate for you again with the air track.

我向你们提到过,重心的运动是稳定的,若不受外力，我可以再次演示,在空气轨上。

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