Snowboarding Physics

The scrape of the snowboard on ice is heard from afar. The 2022 Winter Olympics are underway, with the snowboard halfpipe finals happening earlier this week. Flying 20 feet into the air, Ayumu Hirano has landed the first ever triple cork in Olympic snowboarding halfpipe history. The triple cork, an extremely dangerous and committing trick, consists of three off-axis flips with a varying degree of rotation.

A trick at that level can only be done with extreme precision. However, the way athletes achieve this incredible feat is due to their small moment of inertia. Before diving into inertia, the conservation of angular momentum must first be addressed. The speed of a rotating object is constant until it is acted upon by an external force. Therefore, in order for an athlete to spin as fast as possible, they must decrease the external force as much as possible. A way to do so is to decrease their moment of inertia. A moment of inertia is a quantity expressing a body's tendency to resist angular acceleration. The moment of inertia is just the mass times the square of perpendicular distance to the rotation axis, I = mr^2. A lower moment of inertia can be a result of physically making the body smaller by perhaps pulling limbs tight against the body or crouching down. Furthermore, the conservation of angular momentum demonstrates the increase of speed when an object is smaller. Angular momentum, denoted by mass*velocity*radius (distance from center to point) (This can be applied to a human body by summing the results of all the particles in the body). If one variable decreases, then another must increase because angular momentum is conserved. So, if an athlete decreases the radius of their body by tucking tightly into a ball, then the velocity must increase (mass cannot increase) making them spin faster (1). In this case, Hirano displayed both of these techniques, ensuring that the triple cork was possible due to his compact frame.

http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html

(1) In reality, the increase is because the centripetal force and the direction of motion are no longer perpendicular. The centripetal force has a component in the direction of the motion when moving inwards, causing the rotational velocity to increase.