If you have been watching skateboarding at the Olympics, you may be wondering how the skaters manage to keep going up and down ramps long after friction should have consumed their initial gravitational potential energy.
That process is called pumping, and most skaters will learn how to do it by going back and forth on a half-pipe. If you are not familiar with the lingo, a half-pipe comprises two ramps that are connected by a lower (sometimes flat) middle section. A good skateboarder can skate up the side of a ramp, turn around and do the same on the other side – and continue to oscillate back and forth in the half-pipe.
What’s obvious about the physics of this scenario is that the gravitational potential energy of the skater while at the top of the half-pipe will be quickly lost to friction. So how does a skater keep going? How do they pump kinetic energy into the system?
Variable pendulum
It turns out that the process is similar to an obscure way that you can keep a playground swing going – by standing on the seat and shifting your centre of mass by squatting down in the centre of a swing and rising up at both ends of a swing (see video below). This can be understood in terms of a pendulum with a length that varies in a regular way – and that is how Florian Kogelbauer at ETH Zurich and colleagues in Japan have modelled pumping in a skateboard half-pipe.
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Their model considers how a skilled skater modulates their centre of mass relative to the surface of the half-pipe. Essentially this involves crouching down as the skateboard travels across the flat bit of the halfpipe, then pushing up from the board during the curved ascent of the ramp. Pushing up reduces the moment of inertia of the system, and conservation of angular momentum dictates that the skater must speed up.
The team compared their model to video data of experienced and inexperienced skaters pumping a half-pipe. They found that experienced skaters did indeed adhere to their model of pumping. They now plan to extend their model to include other movements done by skaters during pumping. They also say that their model could be used to better understand the physics of other sports such as ski jumping.
The research is described in Physical Review Research.
And if you are interested in the physics of the playground swing, check out the video below.
