With Boston being selected to make a bid for the Olympics, I was reminded of possibly one of the more remarkable Olympic sports ~ aerial skiing.

Arial skiers hit their ramps in excess of 40 miles per hour and jump more than 50 feet above the ramp to accomplish complex aerial maneuvers. These athletes rely on fairly simple properties of physics, known to scientists and engineers as angular momentum and the moment of inertia, to control their maneuvers. To understand how they manage their twists and amazing spins, think for a moment to another Olympic sport ~ figure-skating. When figure skaters move into an in-place spin, they control their speed of rotation by moving their arms closer to or further from their bodies. That is, they are changing their moment of inertia (increasing the moment when they tuck their arms towards their bodies) and relying on the conservation of their angular momentum to handle the rest.

Conservation of angular momentum is a fancy way to say one of Isaac Newton’s fundamental rules that something is going to keep moving unless a force is applied to it. While practicing figure skating can be done with your feet firmly on the ice, perfecting aerial ski jump motions at high speed and so far above the ground can be a very dangerous activity. So how do Olympic free-style skiers practice?

Most aerial skiers practice by jumping into a pool. They can do this very safely and with minimal chance for injury. They practice their jumps hundreds of times before even moving to the snow. But as a friend recently told me, the water ramp wasn’t always the safest way to learn. Early in the use of the water ramp, trainers found that their skiers were still being injured far too often in practice sessions. In some cases, skiers were injured more often than on traditional snow ramps. To figure out why the water ramp wasn’t reducing injuries as expected, an engineering team was brought in to examine (and improve) the water ramp.

The engineering team solved the problem by placing perforated air tubes at the bottom of the pool. The tubes allowed for a constant stream of bubbles to come up through the water. The solution addressed two key factors of the landing area: surface tension and density. Anyone who has ever belly-flopped off the diving board at a local pool is familiar with a property of fluids known as surface tension. This happens because on the surface of a fluid, the individual molecules are predominately pulled in tension with other molecules on the surface, creating something of an invisible net.

The other problem with liquid water as opposed to the snow on the ski jump was density. The density of snow is far less than the density of water. In some cases, snow can be more than 10 times less dense. The addition of the bubbles simultaneously decreased the density of the pool and broke the surface tension, allowing aerial skiers to practice more safely.

So while the Summer Olympics are on in 2024 (maybe in Boston), you can think about all those aerial skiers practicing ~ even though there’s no snow.

By R.J. Linton