Extreme rollers: physics helps two adventurous skaters pull off amazing stunts

Science World, Nov 22, 2004 by Britt Norlander

When daredevils Jean-Yves Blondeau and Dirk Auer strap on their inline skates, they plan on more than just a leisurely glide in the park. For one, Blondeau doesn't attach wheels just to his feet: He lashes 28 of them to his whole body. "I have wheels on my belly, back, elbows, knees, hands, and feet," he says.

Blondeau spent years researching and designing his "Buggy Rollin" suit. Now, he can cruise the roads lying on his back or even facedown on his stomach with his nose less than l0 centimeters (4 inches) from the ground. "Just standing on skates wasn't enough fun for me," he says.

Unlike Blondeau, Auer limits his wheels to his feet. Still, his skating stunts are far from ordinary. With the help of an engineer, he designed a device to fasten his skates to the rails of a roller coaster. Then, supported by a safety crew, Auer skated a 520 meter (1,706 feet)-long coaster. "I was a human roller-coaster car," he says. "Nobody had ever come up with such an insane idea before!"

They may be extreme, but these high-tech rollers use simple laws of physics--plus a watchful eye from trained safety experts--to pull off extraordinary stunts.

GOING DOWN?

Whether standing on roller-coaster rails or lying facedown, Auer and Blondeau both strive to maximize speed. Normally, skaters build forward velocity (speed in one direction) by exerting a force (push or pull) on the road. According to Newton's third law of motion: When a skater pushes off with one leg, the ground reacts-pushing the skate forward with an equal but opposite force. With each backward leg pump, the skater's forward velocity increases.

To max out his forward velocity, however, Buggy Rollin' Blondeau rests his legs. Instead, he lies facedown at the top of a hill. There, his body holds gravitational potential energy (stored energy due to height). With a safety team nearby, he drops headfirst downhill. His stored potential energy converts to kinetic energy (energy of motion), and he torpedoes down the slope. "My record speed was 98 kilometers (61 miles) per hour on a steep downhill road in Italy," he says.

Auer uses the same physics to cruise the roller-coaster rails. "I began at the starting ramp--at a breathtaking height of 27 m (89 ft)," he says. The kinetic energy he gained while rolling along e ramp helped him rocket down the coaster's mils. "I did the whole run in 58 seconds at a top speed of 65 km (40 mi) per hour," he says.

WHAT A DRAG

Racing downhill, both extreme rollers feel the air whoosh in their faces. While the wind can be thrilling, it also slows the skaters' rides. "As [their bodies] move through the air, they have to literally shove air molecules out of the way," says Steven Manly, a physicist at the University of Rochester. That causes air resistance, a slowing force. And thanks to Newton's third law, "as you push the air forward, it pushes you backward equally hard," says Louis Bloomfield, a physicist at the University of Virginia.

One way to reduce air resistance: Shrink the surface area exposed to the wind. Then, the skaters push fewer air molecules out of the way. That's why Blondeau's favorite position is flat on his stomach, a move he dubbed the "torpedo." Instead of smashing into air molecules with his entire body, only the top of his head and shoulders collide with them.

Similarly, to slash air resistance while rolling on the rails, Auer tucks his body and bends his knees. He also wears a special helmet. "It's pointy [in the front], so it helps him spread air molecules sideways and out of the way without slowing him down as much," says Manly.

TIGHT TURN

A streamlined helmet helped Auer hit the straightaways at top speed. But when he reached the coaster's 13 curves, he struggled to stay on his feet. To bend him around the track, the rails exerted a centripetal force (force that causes an object to move in a circle) on his legs. Meanwhile, thanks to Newton's first law of motion (see Nuts & Bolts, below), his body tried to continue on a straight path. That made him feel as if his body was being smashed sideways. "It's the same feeling you have in a car when you go around a tight curve," says Manly. "You feel like you're being thrown against the car door."

The centripetal force was stronger when Auer rolled faster. "You need to have a lot of experience to balance," he says. To keep from crashing, he leaned into the turns. "That way his weight feels like it is directly over his feet," says Bloomfield.

At the end of the run, Auer says his legs trembled from trying to hold himself up: "I couldn't stand the forces for one second longer."

Nuts & Bolts

Newton's first law of motion states that an object has inertia: It resists change in its motion. So an object at rest remains at rest, and a body moving at a particular velocity continues at that velocity unless a force acts on it. This inertia keeps Auer's body moving in a straight line when he hits the coaster's curves. To turn him, the rails exert a force on his body.

DID YOU KNOW?

* Inventor John Joseph Merlin is thought to be the first creator of roller skates. He designed the skates which had in-line wheels--in the mid-1700s. Unfortunately, he wasn't a good skater. He wore them to a party in London, England. Unable to stop rolling, he crashed into a mirror and seriously injured himself.