Skateboarding is hard.
When I was about 10, I broke my first skateboard by riding it into a ditch. A decade later, in college, I broke another skateboard within an hour of owning it (surely a record) in a short-lived attempt at doing an ollie. (Surprisingly, the store accepted a return on that board even though it was in two pieces.) Then I was gifted a really nice, high-quality skateboard. The first thing I did with it was ride it down a big hill, a valiant but ill-fated adventure which ended with me jumping off the skateboard, rolling down the grass, and arriving scraped up, deflated, and rather disoriented near the entrance to my college cafeteria. (In my defense, the wheels and ball-bearings on that skateboard had been pre-lubricated to minimize friction, and why would anyone do that, that’s just crazy.)
So believe me when I tell you that I am incredibly envious of skaters who can pull off tricks like this.
Now, I might not be able to skate to save my life, but I can do a little physics. So here’s a thought – maybe I can use physics to learn how to do an ollie. Here’s the plan. I’m going to open up the above video of skateboarder Adam Shomsky doing an ollie, filmed in glorious 1000 frames-per-second slow motion, and analyze it in the open source physics video analysis tool Tracker.
The first thing I did was track the motion of the front and back wheels (Tracker has a very convenient autotracker feature that can do this for you.)
One useful physics trick here is to track the center of mass of the skateboard, i.e. the average of the positions of the front and back wheels. Here is that curve overlapped in green.
Now, if you were to do the same tracking exercise for a soccer ball that’s been kicked, you’d get a neat arc-like shape called a parabola. This is the characteristic shape you get when the only force influencing an object’s motion is gravity.*
But the green curve in the above gif — the motion of the center of mass of the skateboard — is nowhere close to being a parabola. It’s lumpy and weird. This means that gravity isn’t the only force affecting the skateboard. Unlike a soccer ball in mid-flight, a skateboard mid-ollie is being actively steered.
This is exactly what makes doing an ollie so hard. It’s not enough to get the skateboard up into the air – you also have to steer it while it’s in the air.
In fact, we can work out how you need to steer the skateboard. Tracker has a nice feature that we’ll call ‘force arrows’. These arrows show you how much force acts on an object at every instant, and in which direction the force acts. So for example, if you were to kick a ball into the air, while the ball was mid-flight, this arrow would always point down and be the same length, even though the ball is moving forward. That’s because the only force acting on the ball is gravity, which pulls it straight down, and acts with a constant strength. (For those of you who’ve studied physics, these arrows denote the acceleration of the center of mass, which by Newton’s second law is proportional to the net force acting on the skateboard.)
Here’s what we find when we work out the force arrows for the skateboard.
Or, if you prefer to see all the arrows overlaid,
It’s a neat piece of science art, and it also tells us something interesting. The arrows show us that the force on the skateboard is constantly changing, both in magnitude as well as in direction. Now the force of gravity obviously isn’t changing, so the reason that these force arrows are shrinking and growing and tumbling around is that the skater is changing how their feet pushes and pulls against the board. By applying a variable force that changes both in strength and direction, they’re steering the board.
In fact, we can go back and see how much force each wheel experiences.
Crucially, at any instant, each foot applies a different amount of force. These unequal forces at each end is what causes the skateboard to turn (in physics lingo, it creates a torque). It’s how the skater steers the board.
We can see this more clearly if we subtract away the motion of the center of mass (i.e. subtract the green arrows above from the red and the blue arrows). Now, we’re only looking at how the wheels accelerate relative to the center of the board, not relative to the ground.
You can see there how the skater uses unequal forces to turn the board, shifting their weight from their front foot while moving up, to their back foot while descending.
To summarize, a skateboarder’s feet need to do two things successfully to complete an ollie. They need to provide a changing force to move the board correctly (so that the combined force of gravity and the skater’s feet add up to the green arrows above), and they need to provide different amounts of force with each foot (shown by the red and blue arrows above) to steer and turn the board into the right orientation.
Sadly, after all this geeking out, I’m no more successful in my attempts to do an ollie. But at least now I can explain why I suck at it.
Thanks to Robin Wylie for helping me headline this post.
*Technically this curve is a (segment of an) ellipse, but so long as you aren’t kicking the football into orbit, it’s close enough to a parabola.