By HENRY FOUNTAIN

PARK CITY, Utah - The first time you watch skiers hurtle off a curved ramp at 50 kilometers per hour, 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.

Keep watching and you will quickly have second - and third - thoughts. You begin to notice how the skiers adjust their starting point on the inrun to reach the proper takeoff speed, how they practice odd arm movements, like dolls whose limbs are being manipulated by unseen hands.

It is not fate that plops these freestyle aerialists, as they are called, down at the end of their jumps, more or less upright and safe . 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 during coverage of the 2010 Games in Vancouver, British Columbia being held February 12-28.

It has roots in freestyle skiing . But aerials has developed into a serious discipline that borrows much from gymnastics.

“The forces are pretty simple,” said Adam Johnston, a physics professor at Weber State University in Ogden, Utah, who took a break from his teaching duties one recent afternoon to watch aerialists with the United States Freestyle Ski Team train at Utah Olympic Park, which was built for the 2002 Games in Salt Lake City.

“There’s the force of the ramp on his skis, and the force of gravity on him,” Dr. Johnston said, after Ryan St. Onge, the reigning world champion in men’s aerials and a member of the United States 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 Mr. 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,” Dr. 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.”

To ensure he will have sufficient rotational, or angular, momentum to see him through three flips, Mr. 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 Mr. 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.

During a jump, he does this more or less intuitively because, like other team members, he has spent hours practicing without skis on trampolines and - in the warm months - skiing off ramps lined with a plastic snowlike surface into water.

A proper ramp provides a good start to a jump. If a skier just holds his body still, the rotational momentum will result in a triple flip in layout, or nontwisting, position. While it looks death-defying, that kind of jump is kid stuff to a judge. So aerialists add body twists, rotating about a second axis, one that runs head to toe. In this training jump, Mr. St. Onge adds a full twist in both the second and third flips - a lay-full-full in the language of the sport.

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.

“It’s all about picking that one spot,” Mr. St. Onge said, “and making sure that’s exactly where you’re going to land.”

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, Mr. 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 twice the speed of aerial jumpers.

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