As the new Tony Hawk exhibit at OMSI shows, skateboarders use skatepark features by "pumping," or transitioning their center of mass in ways that propel movement. It's the same way you get a swing to move by moving your legs and torso in opposite directions. Friction is always a factor, which is why skaters prefer new wheels and why parks are always made out of smooth concrete instead of grainy asphalt.
To learn a little more, we asked OMSI educators Matt Miller and Kristi Falkowski to explain the workings of the Burnside Skatepark, under the east end of the Burnside Bridge, which was famously featured in Tony Hawk's Pro Skater, the 1999 video game that made Hawk a household name.
1. Drop-ins: Using energy derived from corn chips and Red Bull, skateboarders climb up a bowl or curved drop-in to start skating. Once resting atop the feature, they have potential energy as gravity waits to act on their mass as they move down the curved slope. Once they drop in, gravity is now acting on their mass and the energy is in motion, it is kinetic energy. Eventually, if no more energy is added by pumping, kinetic energy will run out due to friction, air resistance and some kinetic energy being converted to sound and heat. Then, they will stop.
2. Bowls: After they drop in, centripetal force carries skateboarders around. Centripetal force keeps a body moving along a circular path, directed toward the radius at a uniform speed, allowing the skater to move around the curve without doing additional work.
3. Vert ramps: To get to the top of a vertical surface, skateboarders must match the speed it would take them to fall from the top. If they gain enough speed to ride up the ramp and launch into the air, they can manipulate their angular momentum to spin. Once airborne, skaters will tuck in to create a tight center of gravity and increase rotational speed. If their arms are flailing, leverage effects will displace their center of gravity and slow them.
4. Roller: Skaters adjust their center of mass at ideal times, "pumping" over the curved mass to increase velocity.
5. Pyramid: Pyramids have hard edges and a flat top with lips to pop off of and transitions to "pump" over.
GO: Tony Hawk Rad Science is on display at OMSI, 1945 SE Water Ave., 797-4000, omsi.edu, through May 4. 9:30 am-5:30 pm Tuesdays-Saturdays. $13, free to museum members.