Skateboarders in half-pipes have a need for speed. The faster they go, the higher they can rise out of the pipe. Achieving greater heights is not only impressive on its own, it’s necessary for pulling off acrobatic tricks like Caballerials and McTwists.

On flat ground, the conventional method for gaining speed is to push off with one foot. But half-pipes present a much more elegant option for the speed-hungry skater. It’s called pumping.
To pump in a half-pipe, a skater first drops down into a crouch while traversing the flat bottom of the U-shaped pipe. Then, as she enters the sloped part of the ramp, called the transition, she straightens her legs and rises up. By raising her center of mass just at the beginning of the ramp’s arc, the skater gains energy and thereby increases her speed.

Pumping in a half-pipe is closely related to pumping on a swing. To get the swing to go higher, you lift your legs as you pass through the bottom of the swing’s arc, then drop them at the top of the arc. Each time you do this, you gain a little energy and swing a little higher.

From a physics point of view, the extra speed that comes from both kinds of pumping is a result of the equivalence of work and energy. As you move into the bottom the arc, centripetal force makes it harder than normal for you raise yourself. The net work you perform in lifting yourself is equivalent to a net energy gain. This energy gain translates into extra speed and greater height at the top of the swing or ramp.

See for yourself how pumping gets skaters to fly higher. All you need is some string and a coffee mug.