A hula hoop rotating around a person’s waist is an example of circular motion where the centripetal force on the hoop is directed towards the centre of the circle rather than the centre of the hoop. The friction force on the hoop is relatively small by comparison since the hoop rolls around the waist. A slight complication is that the hoop rotates in an elliptical path if the waist rotates back and forth in a straight line or if the waist rotates in an elliptical path. Three simple experiments are described to illustrate the physics involved.

中文翻译：

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呼啦圈的物理

绕人的腰旋转的呼啦圈是圆周运动的一个示例，其中，向箍施加的向心力指向的是圆圈的中心，而不是箍的中心。相比之下，由于箍绕腰部滚动，因此箍上的摩擦力相对较小。稍微复杂一点是，如果腰部沿直线前后旋转或腰部沿椭圆形路径旋转，则箍沿椭圆形路径旋转。描述了三个简单的实验以说明所涉及的物理学。