Masters of their Center of Mass

WOW! This new video of trick bike rider Danny MacAskill tearing it up around his hometown of Edinburgh is absolutely incredible.

Just as unbelievable is this video of unicycling champion Kaori Matsuzawa showing off her moves on only one wheel.

On the one hand, both of these videos show some pretty gnarly moves by their riders. With a repeat watching though, and an eye towards the physics of what’s going on, you can see how these amazing professionals are able to pull off some of these tricks.

The key in both of the videos is balance. Some of the most amazing moves both MacAskill and Matsuzawa pull are when they’re hardly moving at all, and just balancing on an almost stationary cycle. It takes years of practice, but the way they can stay up there like that is by perfectly aligning their body’s center of mass over their bike’s center of mass, supported over a stable surface. A center of mass is the position in an object around where all weight is evenly distributed. You can see MacAskill’s center of mass, when he backflips off of the tree. It’s the axis around which he and his bike rotate around as he flies through the air. It’s a single point because he’s firmly attached to his bike forming a single object, rather than two separate points, one for him and one for the bike.

In the second video, you can see Matsuzawa spinning around like a ballerina. Even though her center of gravity isn’t supported, the angular momentum of her spinning creates a rotational force (torque) that stabilizes her. It’s exactly the same principle that lets a spinning gyroscope stay straight.

Bicycles and unicycles are amazing devices that utilize a tremendous amount of physics to make them go. Tricks like these can only be done after years of practice. If you do want to start doing tricks, remember to start small, and work your way up over time. And always wear a helmet.

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