Author: Edgar Zhu
Editors: Lydia Ren and Cynthia Zhang
Artist: Selena Zheng
Imagine this: an unprotected tightrope walker—a seemingly madman without any wires on his waist—strolling in a few hundred meters up in the air, the only thing he has is a single piece of a long pole which seems to have nothing to do with safety precautions. This is a familiar scene in the movie Man on Wire. The protagonist, Philip, finished what is still a stunning feat, standing between the World Trade Center towers in New York City, 110 stories and 412 meters tall. Philip, kneeling or lying on the cable, moved slowly and danced along the way. And he finished the whole process only with the help of a long metal pole. Behind this, however, certain scientific theories are preventing Philip from falling from a hundred feet up in the air.
The main reason why Philip was able to hold his balance is because of the enormous rotational inertia the long pole has. Rotational inertia can be comprehended as the inertia of an object in a rotational motion. Besides the proportional relationship between mass, it also has a positive relationship with the distance from the mass points to the pivot. Most iron poles that acrobats hold concentrate their mass at the two ends of the pole. This provides them with enormous inertia, the base of the whole process. Rotational motions require a large force to move. The more rotational inertia a pole has, the more force it will need to be accelerated. When the gravitational force acting on the pole is constant, the longer the pole is, the smaller the angle the pole will rotate. In that case, acrobats will have enough time to adjust their balance and the position of the pole.
Another peripheral reason why a long pole can help acrobats keep balance is that the pole helps them lower their center of gravity. Because of the mass held at the ends of the pole, it sometimes bends a little bit to the ground. Now we can see the acrobat and the pole as an entirety, so their total center of mass is lowered. As the center of mass is closer to the wire the man stands, it's easier for him to keep balance. After looking at a brief explanation of why a steel-wire acrobat can balance on an ordinary pole, maybe you could place a pole half a meter up in the air in your backyard and start walking on it.
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Baidu Baike. "转动惯量." 百度百科_全球最大中文百科全书, baike.baidu.com/item/转动惯
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