So I mentioned to you that the motionof 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。
2
So now we're going to turn off the lights and I want to show you the motionof the center of mass.
我们要关上灯，我想给你们展示，质量重心的运动。
3
And ultraviolet light will interact with that little ball, the centerof mass, and when I then make it dark, you will only see the motionof the center of mass.