At the 2019 US Figure Skating Nationals, 13-year-old Alysa Liu successfully completed a total of three triple axels. Most women who do this jump in competition are in their twenties. It’s jaw-dropping jumping feats like this that always prompt spectators to ask, “How does she do that? How is that possible?” Personally, when asked about how I jump on the ice, I’m at a loss of what to say. All skaters know what they’re doing, but when we’re learning a jump, many times we just kind of throw ourselves in the air and hope for the best, which sometimes doesn’t end too well.
But looking at it from a scientific perspective, how does a skater jump and skate? I haven’t taken AP Physics C, so the science of figure skating is in very simple terms to me, but it is still very interesting for anyone to know.
The moment of inertia plays a large part in figure skating. The moment of inertia is a measure of how much a body resists angular acceleration, which acts as a form of resistance. The moment of inertia is calculated based on mass times the distances from the radius squared. When attempting a double or triple axel, a skater wants to decrease their moment of inertia. If the skater can reduce the average radius, which is the average distance of their mass to their axis of rotation, then their moment of inertia is decreased; this increases their angular velocity, meaning that the skater rotates faster. When a skater jumps, they pull in their arms and cross their legs tight. By doing so, this radius is shortened. The moment of inertia is decreased, and the skater spins faster in the air because of a higher angular acceleration.
A skater must rotate fast to finish all the revolutions in the air so the jump is not cheated (short on revolutions) and to make the jump look easy and get a good grade of execution which is the grade the judges give on how well the element was completed. Alysia Liu would not have been able to complete all three and a half revolutions of a triple axel if she hadn’t pulled in tight and had a decreased moment of inertia.
Also, when the skater performs certain positions in spins, like tucking one leg and bending over, they are squeezing themselves to be as small as possible. They are also reducing the moment of inertia to spin faster so the spin goes around the required amount of times.
Weight also a affects inertia. The more mass the person has, the more inertia, or resistance to change, their body provides. Thus, skaters need to be light, and if the skater has more weight, then it will be harder for the skater to complete all the revolutions in the air. In a documentary Alina Zagitova and Evgenia Medvedeva, the 2018 Olympic Gold and Silver figure skating medalists, respectively, discussed how people believe they can eat anything they want because they burn so many calories skating. In reality, they have to strictly limit what they eat for their jumps.
Another part of physics that lies behind figure skating is the center of mass, which is the weighted center of the body. Skaters have to keep their center of mass over their skates in order to be stable. My coaches always tell me to imagine stacking my head on top of my hips, or to imagine I’m a doll with strings attached to me, and the string on the head is stretching as high as it can go. When jumping, if the skater doesn’t center their mass over their landing leg, then they’ll most likely not be able to land the jump because their body is not prepared. Liu’s air position was straight and she was right over her landing leg, allowing her to land the triple axels successfully.
Even if the skater doesn’t know what’s scientifically going on while they’re skating, the science is everywhere. The moment of inertia, weight, and the center of mass play key roles in figure skating. To do what they do, skaters practice a lot, utilizing physics to efficiently rotate and maintain stability.