Thrills, chills and spills: what keeps two daredevils safe on the slopes? World-class skills and an expert grasp of Physics

Science World, Jan 10, 2003 by Nicole Dyer

Picabo Street is one of America's greatest downhill ski racers. She barrels down snowy slopes at speeds up to 80 miles per hour clad in little more than spandex and a helmet. "Speed is in my system," says Picabo (pronounced Peek-a-boo). "I drive fast, talk fast, ski fast." Her mastery of the sport has earned her an Olympic gold medal and 11 World Cup championship titles.

Rebel extreme skier Glen Plake doesn't compete for prizes or titles, but he may be the world's most recognized freestyle skier, thanks to gravity-defying stunts and a trademark 17-inch bleachblond Mohawk. And at age 38 his skills have never been sharper. "I'm in incredible shape," he says. "It's my new calling to let the world know you can hammer for a very long time."

Read on to learn how these two thrill seekers use physics to dominate the slopes.

IT'S ALL DOWNHILL

Any skier can race down a mountain and pick up hair-raising speed thanks to gravity, a downward force that acts on all objects. "Gravity is the hook," says physicist David Lind, author of The Physics of Skiing. "It's the main force acting on a skier." But let gravity alone do all the legwork and you may find yourself careening down a slope like a runaway train. To tame the speed of your descent--and make it to the bottom in one piece--you need to turn, or zigzag, down the slope. A quick clean turn not only keeps you in control, it's the secret to faster race times, explains Lind.

Picabo Street knows a thing or two about speedy turns. She's worked on perfecting them since age 9, when she first determined she would one day nab Olympic gold. At 17 she'd conquered nearly every North American junior ski race. And at the 1998 Winter Olympics in Nagano, Japan, Picabo's Olympic dream came to life. She rocketed her way to a gold medal in the women's Super G event, a daunting 600-meter vertical course in Which racers weave around 30 plastic gates rooted in the snow. Whoever completes the turns fastest wins the race. Picabo outperformed the competition by executing each turn at maximum speed--with zero mistakes.

Picabo turns with masterful precision in part because she takes full advantage of Newton's third law of motion: When two objects act on each other they experience equal and opposite forces. So when gravity presses a skier into the snow, the snow exerts an equal and opposite reaction force on the skier.

If done expertly, her technique--called carving--leaves a smooth S-shape track in its wake and sacrifices less energy to skidding and vibration. "True carving without skidding results in the optimal ski run," says Lind. "It's the edge of the ski that's biting into the snow and holding you in place."

A FORCE TO RECKON WITH

Force, of course, is essential to skiing: Without it a skier would be unable to change the direction of her motion. Newton's first law of motion describes it this way: An object moving at a steady velocity (speed and direction) keeps moving at that velocity unless acted on by an outside force. In the skier's case, the outside force comes from kinetic, or moving, energy generated mostly by a skier's own leg muscles.

But apply too much force--or too little--and disaster can strike. In March 1998, during a World Cup downhill race in Crans Montana, Switzerland, Picabo careened off a jump at 60 mph. She landed too far back' on her skis and threw off her center of gravity, the point at which the weight of an object balances; as a result, she was unable to shift kinetic energy into her skis. With no outside force acting on the skis, Newton's first law of motion turned from friend to foe: Picabo's body continued to fly in the direction of her velocity--straight into a fence. The crash shattered her thighbone, or femur, into nine pieces. "I went from 60 mph to zero in two feet," says Picabo. "It's that loss of control, that moment when you know you're going down, that's terrifying."

"HAIR" BORNE

Like Picabo, Glen Plake started skiing as soon as he could walk. But Plake has never won a World Cup nor competed in the Olympics. Racing against the clock is simply not his thing, he insists. Instead, the Lake Tahoe, Nevada native finds glory in freestyle skiing. "It's the creative side of the sport," Plake explains. "Creative" to Glen Plake means finding adventurous ways to battle mogul fields (steep slopes puckered with endless one-meter bumps), or survive midair maneuvers such as the 720-degree twist known as the Rodeo-7.

His death-defying specialty: the cliff jump. Plake routinely launches himself off 12-meter rocks and makes picture-perfect landings on wall-steep slopes. How? Consider The Law of Conservation of Momentum: When two objects collide, momentum--an object's mass multiplied by its velocity--is transferred from one object to another. For example, when Plake soars off a cliff his momentum (165 pounds of body weight x 35 mph) is transferred into his landing. If the slope is steep enough, his momentum transfers speed into his downward descent. If the slope is too fiat, momentum transfers directly into his legs. "You can compare it to a NASCAR crash," Plake says. "If a car hits a wall dead on, the driver gets hurt. But if he keeps going, he usually escapes injury."