A new sport arrived on the Olympic scene in 2021: skateboarding.
On Tuesday, Japan’s Sakura Yosozumi won gold at the final women’s skateboarding event. The last men’s event will air at 10:00 pm EST on Wednesday. Both competitions center on the second of the two Olympic skateboarding disciplines: park, as compared to last week’s street.
During the competition, skateboarders are judged on the quality, difficulty, and originality of their tricks, flips, and rotations. In park, specific moves include hand plants, grinds, alley-oops, and grabs.
These tricks seem to defy the laws of physics, but it’s quite the opposite: These athletes have learned to harness physics to jump, flip, and somehow keep the board aligned under their feet.
How do they do it? Inverse asked Bill Robertson, also known as Dr. Skateboard, for the low-down. Robertson is a professor of STEM education at the University of Texas at El Paso. His specialty is teaching physics through — you guessed it — skateboarding.
An equal and opposite reaction — Let’s start with the basics. Even just to ride a skateboard across the flat ground, you need some physics. When a skateboarder keeps one foot on the board, while using the other to push down on the ground, they propel themselves forward with the same amount of force that they’re pushing backward. It’s Newton’s third law: For every action, there’s an equal and opposite reaction.
The same tenant is invoked when the skateboarders “pop,” or jump off the ground with their board perfectly in tow. It’s the forceful push downward on the board (and the board’s subsequent contact with the ground) that launches it back in the air.
The most basic skateboard move is the ollie, where skaters jump off the ground with the board and land back down. Part of keeping the board underfoot is timing, that is, making sure the skater’s jump is in line with the board’s trajectory. Skaters use their front foot to level out the board to make their landing pad. (An ollie was one of the moves seen during Olympic street competitions — for example, Team USA’s Jagger Eaton pulled off a backside 360 ollie.)
Any time the board is in the air, explains Robertson, it’s at the mercy of the same forces as an airplane: lift, thrust, drag, and weight. Thrust is what moves the board forward, lift is what moves it upward, drag from the air pushes backward, and weight is gravity’s way of pulling it back down.
The speed of the take-off (and the jumping ability of the skater) affects the amplitude, or height, of the jump, which takes the shape of a parabolic arc.
“At some point, gravity’s going to win, no matter how fast you’re going,” Robertson says.
How to land on a skateboard, with physics
The key to a good landing is keeping your feet over the wheels, Roberston explains
“When skaters make tricks with big ollies, like going down a stair set or down a drop, they always talk about ‘landing bolts,’” he says.
When you’re looking at a board from above, you can see the bolts where the trucks and wheels are attached. Keeping your feet on the bolts, especially during a high-force landing, is critical. If you have your feet too far forward or too far back, you’ll tip.
Is the stance too wide altogether? If so, “the board can snap because there’s so much force,” Robertson says.
In addition to keeping your feet aligned with the board, there’s also the challenge of not falling over yourself. That’s where your center of gravity comes in.
“A lot of times your own center of gravity is a spot a little below your belly button,” Robertson explains. “But when you’re on your board, it’s even a little lower, depending on what you’re doing.”
“Even though there’s all this motion going on with a skater’s arms, or legs, if you look, usually their middle is right over the board,” he says. “If you’re quieter in the middle, keeping that center of gravity over the center of the board while it’s doing its thing, you have a much better chance of landing tricks.”
Of course, skaters may very well not be thinking about their moves in these terms.
“You don’t really stop and think about it like, oh I better get my center of gravity over my board, you know? But if you look at good riders, that’s what happens,” Robertson says.
Skating through centripetal forces — When skaters execute a trick, you’ll notice they shrink themselves. “Watch good riders when they do something, they compress closer to the board, because it’s easier to manage,” Robertson says.
Part of this is to bring their center of gravity closer to the board’s, which helps with control. But when any spinning is added to the mix, centrifugal and centripetal forces are at play.
Any time you’re spinning around, centripetal forces push you in, while centripetal forces pull you out. The latter is easier to feel — it’s why your arms or poofy skirt fly outward while you’re spinning.
For skaters, “you’re trying to keep those forces in line,” Robertson says. “One of the ways that people do that is using what’s called the moment of inertia. It’s a way of slowing you down or speeding you up.”
Perhaps the most classic moment-of-inertia users are figure skaters. You’ll see them pull their arms in tight when they’re spinning fast, then release their arms when it’s time to stop. It’s also easy to see in spinning gymnasts and divers — the more laid out they are, the slower the spin will be.
This also explains why flipping the board on some axes is easier than others.
You can try it yourself with an object like your phone (thanks Physics Girl aka Dianna Cowern on YouTube for this trick!) If you toss your phone a few inches in the air, flipping it along its longest axis, it’s fairly easy. That’s the skateboarders’ basic kick-flip. Now, keep it level, but spin it front to back, like you’re tossing a pizza. It’s a little harder but still works.
“Skateboarding is definitely an international phenomenon.”
Now try to flip it back over-front, like a somersault. While it’s in the air, it’ll twist, thanks to what’s called the intermediate axis theorem.
Flipping about the mid-axis of any object is inherently unstable because it takes so much less energy to flip around the longer axis — it has a different moment of inertia. The equivalent skateboard trick is named “the impossible” for this reason, but all-star Rodney Mullen figured it out in 1982 (the secret, he told Cowern, is guiding the board’s flip with your foot.)
The Inverse analysis — There are countless laws of physics that apply to skateboarding. We could talk about the potential energy a skater has when they’re stalled at the top of a ramp, the mechanical energy they have on their way down, or the kinetic energy they have at the bottom.
We could talk about how skateboard wheels come in different hardnesses, which increase (for beginner skaters who want more stability) or decrease (for pros who want to pull more tricks) friction between the wheels and the pavement.
These were all displayed at this year’s Olympics for the first time. We’ll get to watch again in 2024 in Paris.
“Skateboarding is definitely an international phenomenon. I’ve been all over the world, skateboarded all over the world, and wherever I’ve gone, there have been good skateboarders there,” Robertson says. “I think the Olympics is just going to add to the popularity.”