Number theory and neuroscience are probably not the first things that come to mind when you contemplate the circus. Yet along with the clowns and the fire-eaters, there’s a place for science under the big top. Performers and their coaches have to know, either instinctively or consciously, what the laws of physics and biology will permit them to do; moreover, a surprisingly large number of scientists are themselves circus fanatics, and a few are even performers. Perhaps it’s because the circus proves that science doesn’t stop at the lab door.
The crowds are always awed when the acrobats with Montreal’s Cirque du Soleil start twisting their bodies as they flip. But actually such twisting is, in a purely physical sense, easy. Any object resists efforts to turn it--this is the quality physicists call rotational inertia. Rotational inertia increases as the mass of an object is spread farther from its center of rotation: this seems obvious if you consider that a rotating object (or body) with long extremities has to sweep out a bigger circle than one that is more compact. Covering that extra ground requires more energy.
A performer can describe a circle by rotating around either a horizontal axis running through his hips (he can flip) or a vertical axis running from his head to his feet (he can twist). But his body is centered around those two axes differently. From the axis through his hips, his body might stretch three feet, giving him a high rotational inertia. From his vertical axis, however, his body may extend only a few inches, imparting a low rotational inertia. Thus, in a sense, his body wants to twist more than it wants to flip. You can see this by taking a normally proportioned book and flipping it in the air--it’s almost impossible to prevent it from twisting. What makes twisting such a hard skill, in fact, is that it is too easy: if a performer can’t control it, his landing will be ugly.
