The Physics of Acrobatics
Breathtaking flips, precision balancing, a bit of brawling and swing dancing—it's all part of the show as renowned French circus collective Compagnie XY brings their latest production to Lincoln Center, Il N'est Pas Encore Minuit (It's Not Yet Midnight). Calling Compagnie XY a circus is a bit of a misnomer; it's more of a contemporary hybrid, since the troupe combines elements from theater and dance with spectacular acrobatic feats—and even a spot of science. It's often said that all those dazzling flips and balancing stunts seem to defy the laws of physics. But, in fact, it's basic physics that makes them possible.
Humanity's struggle against instability and imbalance, and how coming together as a community can combat that, constitutes a major theme of the production. That theme is embodied in the many times the troupe members balance on each other's shoulders, stacking as high as three or four people and creating formations akin to a tiered wedding cake. It's also reflected in the physics. The move requires aligning one's center of mass—the point where gravity acts on the body as a whole, typically located about one inch below the navel—precisely over the bottom person's center of mass.
Think of a seesaw balanced across a triangular base; the seesaw is a lever, pivoting back and forth above a fulcrum (the base). The center of mass is the point at which the board is perfectly balanced atop the base, its mass divided into two equal halves. Move that point ever so slightly in either direction, and one side will have more mass than the other, tipping the board off balance such that the heavier end slams into the ground.
Maintaining that alignment may sound simple in concept, but in practice it requires a great strength and skill. The acrobat on the bottom is the fulcrum upon which the other's balance rests. Every slight wobble, slip, or imperfection increases the risk of falling off. Now consider that the performers are frequently flipping into that position from the ground, or being thrown from one set of shoulders to another. Yet they always land precisely where they need to be, making only the smallest of adjustments.
It's hard enough to do this with just two people. Compagnie XY's innovative choreography ups the ante even further by stacking successive layers. The more people are stacked on each other's shoulders, the more of those small slips and imperfections there are likely to be, and the harder it will become to maintain that balance.
Think of building a tower of shot glasses by balancing them on top of each other. At some point, there are so many levels that if the alignment is even slightly off on just one base or rim of a glass, the whole structure can come crashing down. The same principle applies here. And take a moment to feel for the performers at the base, who—when the stack reaches four people high—must support several hundred pounds in weight, similar to what an Olympic gold medalist must lift.
Nor do the acrobats forming the base make it easier on themselves by standing still; they walk around in stacked formation. Technically, walking is a process of constant falling: every time a foot comes off the ground and steps forward, that person is momentarily off balance, falling forward and righting him- or herself before taking the next step. That means when the stacked tiers of acrobats slowly walk around the stage, the center of mass of the person on the bottom is constantly shifting, and the one balancing on the shoulders must readjust accordingly, in real time, without disrupting the performance.
These same principles are also at play when a performer elegantly balances on a cohort's head using just one arm, while his or her legs move into a split. The degree of strength required to pull this is off is staggering, but thanks to physics, it's not impossible. Those splayed legs are keeping them balanced by distributing the body mass more evenly in each leg, just like the equal halves of the seesaw balancing over the fulcrum. This makes it much easier to self-correct to keep that center of mass right where it needs to be. It's similar to the balancing birds sold in many toy stores: there are weights in each wing distributing the weight to each side, and this makes it easier to balance the toy bird on a finger.
Every member of Compagnie XY is skilled at basic flips and twists, but sometimes they get a boost from their colleagues by standing on one end of a long flat board (a rudimentary seesaw) while others jump down on the other end, sending their compatriots flying into the air. This is a classic trick in circus acts. Here, the acrobats are exploiting torque—basically a rotational twisting force, like tightening a bolt with a wrench—to launch each other further into the air than they would otherwise be able to reach by jumping unaided.
The further away you are from the pivot point, the more torque will be applied, because torque is highly dependent on weight. That's why a heavier parent seesawing with a much lighter child will position him- or herself closer to the pivot point—to even out their respective torques. An acrobat jumping down onto the board will do the opposite, positioning him- or herself further from the pivot point to generate maximum torque. It's why the ancient Greek philosopher Archimedes once declared that if he were given a place to stand and a long enough lever, he could move the Earth. Such is the power of torque.
All this makes the performers in Compagnie XY "intuitive physicists" as well as gifted artists. They might not be consciously calculating forces and trajectories in their heads, but years and years of hard-earned experience have given them a finely honed sense of the forces at work, and just what they need to do to exploit them. (The troupe spent 22 weeks developing and rehearsing this new production alone.) Understanding a bit about the underlying science should only add to the sense of awe and wonder at the skill and artistry on display.
Jennifer Ouellette is a science writer and editor based in Los Angeles, and the author of four popular science books, most recently Me, Myself, and Why: Searching for the Science of Self.