有趣。简单地説，球的轉速不變，而轉盘上跟球的接触點的速度靠盘心越近就越慢。

想像一下即可。

The hollow ball has its mass concentrated on its surface at the contact point, therefore it moves quicker than the solid ball since it requires no torque to exert the effect on its interior mass like the solid ball does.

Why always the same ratios 7:2 and 5:2?

Imagine the disc being infinitely large, you will see the size of the ball is not an effectual factor, only the configuration matters.

以上是用常识常理來解釋。要定量解释則要計算，例如為什麽是5：2 和 7：2，軌迹是否正圓，等等。