The physics of freestyle aerialists

NYTimes.com : Freestyle aerialists, skiers that hurtle off a curved ramp at 30 miles per hour, soaring six stories in the air while doing three back flips and up to five body twists, are not actually throwing caution 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."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, 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.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 center of mass as a perfect parabola through the whole thing. From the physics point of view, that’s one of the beautiful things.”