Up in the air and down,with a twist

Freestyle aerialists,as these athletes are known,are not actually throwing caution,along with themselves,to the winds. It is not fate that plops them down at the end of their jumps,more or less upright and safe,in a cloud of powdery snow. It is physics,and plenty of preparation.

Aerials,in which skiers are judged on how stylishly they perform their flips and twists and whether they stick their landings,has been an Olympic medal event since 1994 and will be featured in prime time this month at the 2010 Games in Vancouver,British Columbia.

It has roots in freestyle skiing,the devil-may-care approach to the sport that started catching on in the 1960s and ‘70s. It is two parts hot-doggery to one part Nadia Comaneci,with Isaac Newton keeping everybody honest.

“The forces are pretty simple,” said Adam Johnston,a physics professor at Weber State University in Ogden,Utah,who broke away from his teaching duties one recent afternoon to watch aerialists with the United States Freestyle Ski Team train at Utah Olympic Park.

“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. “That’s all there is.”

But 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 that the skiers and coaches have chopped and fluffed for safety.

“Once he’s in the air,the only force on him is gravity,” Johnston said. “You could trace his centre of mass as a perfect parabola through the whole thing. From the physics point of view,that’s one of the beautiful things.”

To ensure he will have sufficient rotational,or angular,momentum to see him through three flips,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,and St. Onge,who is 26 and first joined the ski team 12 years ago,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 is 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.

“We get longer if we’re too fast with a flip,or shorter if we’re too slow,” St. Onge said.

So much twisting can make it harder to keep an eye on the landing area,which skiers try to do to judge their rotation and land without falling back or,worse,pitching forward.

Perfect landings are rare,but so are severe crashes.

“We can crash every jump of the day and not feel sore,” he said.

By contrast,St. Onge said,in “conventional” ski jumping,the Nordic kind,a skier can crash once and feel it for the rest of the year. The reason is speed. He has never been interested in trying that kind of jumping,in which skiers may reach 60 miles per hour.

“It terrifies me,” he said. “Flying through the air at 100 kilometres per hour just seems silly.”