PARK CITY, Utah -- The first time you watch skiers hurtle off a curved ramp at 30 mph, soaring six stories in the air while doing three back flips and up to five body twists, you can't help but think: These people are crazy.
Then you begin to notice how the skiers adjust their starting point on the inrun (the ramp going to the jump) to reach the proper takeoff speed, how they practice odd arm movements, like giant Barbie dolls being manipulated by unseen hands.
Freestyle aerialists, as these athletes are known, are not actually throwing caution, along with themselves, to the winds.
Aerials, in which skiers are judged on how stylishly they perform their flips and twists and whether they stick their landings, will be featured in prime time this month at the 2010 Olympic Games in Vancouver, British Columbia.
"The forces are pretty simple," said Adam Johnston, a physics professor at Weber State University in Ogden, Utah. "There's the force of the ramp on his skis, and the force of gravity on him," Johnston said, after Ryan St. Onge, the reigning world champion in men's aerials and a member of the Olympic team, zipped down a steep inrun, leaned back as he entered the curved ramp until he was nearly horizontal and flew off at a 70-degree angle.
It is enough to create torque that sends St. Onge somersaulting backward as he takes to the air, arcing toward a landing on a steep downslope.
"Once he's in the air, the only force on him is gravity," Johnston said. "You could trace his center of mass as a perfect parabola through the whole thing. From the physics point of view, that's one of the beautiful things."
St. Onge raised his arms entering the ramp, distributing his mass away from his center of rotation, which is near his hips. In physics, he increased his rotational inertia, resulting in more rotational momentum.
The same principle rules sports like figure skating, in which a skater speeds or slows a spin by moving the arms in or out. It is called the conservation of rotational momentum. St. Onge may not be able to recite the related formula - for the record, it is rotational momentum equals rotational inertia times rotational velocity - but he knows what's going on. He will bring his knees up, for instance, on his last flip if he needs to rotate a little more for the landing.
