# Skateboard physicsMay 16, 2013 6:39 PM   Subscribe

I'm watching the X Games on ESPN, and it's the skateboard competition in a swimming pool-like skateboard park. After the initial drop-in to start a run, the competitors do not put a foot down and kick off, yet they seem to sometimes gain speed during the run, depending on their actions. What's going on here, from a physics perspective? How are they adding energy to speed up?

Are they adding energy from their abs and leg muscles, just by bobbing up and down? Or is it all an optical illusion and the starting point is always the highest, most energetic spot?
posted by Cool Papa Bell to Grab Bag (5 answers total)

My guess is that 50% of it is illusion and fancy camerawork, and 50% is torque from the skaters changing their positions on the board.
posted by deathpanels at 7:00 PM on May 16, 2013

It is called pumping. Essentially, the riders can bend their knees on any minor uphill section so that their center of gravity does not actually go up (meaning that they do not slow down in their forward direction). Further, they extend their legs on the downhill section, exerting more force on the ground and increasing their speed.

Generally speaking, the height of the rider's center of gravity does not matter on flat ground but when you add hills you can optimize by adding energy to the system.

If you have ever watched skateboards or bikes on a half-pipe, they do the same thing... without knowing it, you actually employ something very similar when "pumping" on a swing (think about it, you keep going higher, but never push off the ground any more...).
posted by milqman at 7:06 PM on May 16, 2013 [5 favorites]

(From my husband):

Good Gawd! Do none of you skate?!

Its all in the legs. By extending your (their) legs, as they are naturally un-weighted by the ramp, and pressing into the g-forces as you (they) come back down, the forces are translated from vertical to horizontal and the skater accelerates. As skaters get better at it they become very efficient and can maintain what appears to be like perpetual motion. It isn't as easy as it looks and it's incredibly good exercise from the chest down. If you really want those six-pack abs or a nice flat tummy get a board and come on down to the skate park! They'll laugh at first, but later when they see you at the market or picking your kids up from school you'll get "cool" points like no one's business!

Sheesh!... What a Mephite question. Kudos for asking!
posted by hollyanderbody at 7:17 PM on May 16, 2013 [1 favorite]

It's similar to how you swing on a swing. Skaters press down (kind of stand up 'more,' really) on the board into the curve of the surface to convert gravity and downward force to linear momentum through the wheels.
posted by rhizome at 8:06 PM on May 16, 2013

Yep, the beauty of pumping. In pool skating or bmx, a perfect run means not having to ever put a foot down to gain speed (or pedal), you just pump and keep going faster and maintain your speed as much as possible.
posted by mathowie at 10:27 PM on May 16, 2013

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