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They're cramming into your in-tray: links to YouTube videos of urban athletes performing jaw-dropping tricks that seem to defy the laws of physics. Danny MacAskill's trials-biking showreel had attracted more than 7,500,000 views at press time, and that's just one video. We asked Basil Singer, a physicist and extreme-sports fan, to explain the science behind the stunts. As he identified the forces acting on the athletes, we went straight to the performers themselves for insight into what's going on in their heads as they move.
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Skate Stunts
The TrickSkateboarding staple, the ollie: jumping a board high into the air (without a ramp), and landing with it still in place, as if attached to the skater's feet.
The AthleteDanny Wainwright holds the record for the highest Ollie from flat ground at 113cm, but he wouldn't reveal his spring-loaded technique. "The ollie is the first trick you learn, all the others are based on it. I've skated for 22 years; it's just second nature. I don't think about the physics of it."
The PhysicsSinger explains "The secret to this is rotation about multiple axes. From a crouch, the skater accelerates up by straightening his legs. His back foot exerts greater force on the board than his front does, and the board pivots around the back axle. The skater then slides his front foot up the board; friction between his foot and the grip tape pulls the board higher. To raise the back of the board he pushes his front foot down. In the air, the deck rotates about its centre of mass. If he lifts his back foot up in time with the rising tail, he looks stuck to the board."
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Trials riding
The Trick Dropping safely from height (often roofs, railings or the tops of walls), ensuring that the bike always lands on its back wheel.
The Athlete Danny MacAskill advises us to get some training in first. "You don't try to drop off 13ft railings right away, you develop a technique. At a certain height, the landing will be heavy, so you just try to hang on."
The Physics "While the rider is in freefall, gravity pulls him down at 9.8m/s2. This drop is about 0.8s, so when he hits the ground he's doing 17.5mph. Assuming his mass is 80kg, if he were to decelerate in 0.1s he'd hit the ground with a force of over 6,272N. Landing first on his back wheel reduces this force. The rider pulls the handlebars towards his chest, extending his legs and reaching out with the back wheel. He uses the bike as a lever, with the back axle as a fulcrum. This slows the landing to 0.25s, reducing the force to a manageable 2,509N."
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Free Running
The Trick The standing, two-footed gainer: a parkour trick whereby the traceur jumps forwards, performing a backflip in mid-air.
The Athlete Professional urban acrobat Sam Parham explains how to land on your feet. "You travel forwards - but delay the backwards rotation. It's not a blind landing: you spot your landing early, so you know when to open out."
The Physics "If the traceur does a somersault at ground level, he needs to spin quickly as he's not in the air long. If he jumps off a ledge, there's more airtime. After his feet leave the ground, there are no more rotational forces acting on his body and his angular momentum is fixed. But by changing his weight distribution he can alter the moment of inertia. Rotation can be sped up or slowed down by doing this - extending the body increases the moment of inertia and slows rotation. To spin faster, he needs to tuck into as tight a ball as possible."
Basil Singer is supporting the Science: So What? So Everything campaign, which aims to show the importance of science behind everything, including extreme sports
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This article was originally published by WIRED UK
