Why Do People Wave Their Arms Like Crazy When Falling Backwards
Have you ever seen a skier go off a massive jump and some point in the air they appear to panic and start waving their arms around like crazy? Its called “rolling up the windows” and its well known lingo in the ski world albeit a bit of an anachronism (car windows have been automatic for decades). The term comes from the flailing arm motion skiers make in a desperate instinctual attempt to regain balance when flying through the air. The circular movement gets its name from the resemblance of the how people used to roll up a manual car windows back in the day.
It looks ridiculous but rolling up the windows is actually a showcase of advanced biological physics as humans use conservation of angular momentum to recover from an imbalance. Thanks to British educational author and science presenter Steve Mould who just took a deep dive into this biological application of Newton’s third law: for every action, an equal and opposite reaction.
At first glance, it seems like a simple case of conserving angular momentum. When you fall backward, gravity rotates your body counterclockwise around your feet. To counteract this, it feels like you should spin your arms backward. But if you try it, and you will actually throw yourself onto the floor. To pull your body out of the fall, you must rotate your arms into the fall. By accelerating your arms forward, you generate a corrective twisting force, or torque, on your torso.
“It is a biological application of Newton’s third law: for every action, an equal and opposite reaction.”
However, a bizarre paradox emerges. You only get that helpful twisting force while your arms are actively speeding up. The moment they stay at a constant spin, the force vanishes, and gravity takes over again. Because of this, a basic balancing robot can successfully pull itself upright using its arms, but it gets stuck in a hilarious trap: it is left standing perfectly straight, but its arms have to spin like crazy forever. If it just stops them, the sudden braking force will throw it right back on the floor. To safely bring its arms to a halt, the robot has to intentionally overshoot the mark. By leaning slightly past the vertical point, it uses a tiny bit of a forward fall to act as a brake, allowing the arms to wind down to a stop without ruining its balance.
While a basic robot uses a simple three-term equation factoring in tilt, tilt-speed, and arm velocity to manage this, your brain is doing something infinitely slicker. When you are shoved, your motor cortex fires off a rapid reflex signal to your limbs. Simultaneously, it sends a carbon copy of that command to your cerebellum, which houses your brain’s internal physics engine. This biological neural network, trained by every trip and stumble of your life, simulates the near future in milliseconds, sending out lightning-fast corrections to keep you upright. Your flailing arms may look chaotic, but they are the result of an incredibly elegant computational masterpiece.
The next time you see a skier rolling up the windows, take a moment to appreciate the split-second biological masterclass and computational genius in play. Be safe, fave fun, wear a helmet.
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