If a ring is spun around a vertical rod then the ring remains at the same height for several seconds before it starts to fall. The effect is counter-intuitive. Experimental results are presented to show that if the ring is spinning fast enough then static friction will be sufficient to prevent the ring falling.

中文翻译：

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垂直杆上的旋转环

如果在垂直杆上旋转一个环，则该环会在下降之前保持在相同高度几秒钟。效果是违反直觉的。实验结果表明，如果环旋转得足够快，则静摩擦将足以防止环掉落。