By Caroline Bao and Samantha Zhu If you have had the misfortune of skiing during seasons of sparse snow, you may be familiar with the horrifying sound of scraping skis combined with a terrifying sense of losing control on steeper trails. Paired with low visibility and aggressive winds at higher altitudes, the experience can be unfavorable for many. Particularly in southern California, the slippery snow in several resorts can create harrowing sensations of skiing, or rather skating, down ice. Warmer day temperatures melt the snow and cool night temperatures resolidify the snow, packing it into an unpleasant, firm brick, leading to the coinage of the term “California concrete” - describing its hard and icy texture. Types of SnowAside from the notorious “California concrete,” there is a variety of other types of snow skiers encounter on the slopes. The standard snow that any amateur is familiar with is groomed snow—a hard snow packed down by machines overnight to provide a regular and flat surface to ski on. As the day progresses at a ski resort, the grooves left on the slope by the grooming machine will gradually fade away as gradual melting and tracks left by slope goers cause the surface to flatten out and form small bumps. Since turning and new techniques are generally learned on this type of snow, this article will focus on the physic of skiing on groomed snow. Another type of snow is powder—the fresh, airy snow found after a snowstorm. Skiing on this type of snow requires wider skis that distribute the skier’s weight. Since pressure is inversely related with area at a constant force, larger skis allow the skier to “float” over the snow. Ice, which is actually hard packed snow caused by repetitive melting and freezing, is characterized by a bluish glow and a sheen that can intimidate even the most experienced skiers. Skiing over ice often leads to slipping or falling due to the sudden change in friction (and therefore inertia and center of mass). Ski racers, who frequently encounter hard snow, must have very sharp edges on their skis to produce a force of static friction high enough to prevent slipping off of the slope. TurningWhen people start learning how to ski, they are taught the wedge turn, also known as the pizza slice where the tips of both skis are turned inward in a triangular formation, allowing a limited range of motion to maneuver left and right. Because the wedge turn uses both inner edges of the skis, friction is increased and speed is reduced. But at high speeds, wedge turning is dangerous as it puts an increased strain on the knees from torque and increases the risk of injury. A more advanced type of turning is called the parallel turn, where the weight is continuously redistributed on each leg depending on which way the skier is turning. On a left turn, the uphill leg (right leg) holds 20% of the body weight while the downhill leg (left leg) would hold 80% of the body weight in groomed snow. Depending on the type of snow, the weight distribution in parallel turns would change to allow the skier to smoothly cut through it. For carving, a specific type of parallel turn where no skidding is involved, the skis are pointed in the same direction as the velocity, and the radius of the turn equals the sidecut radius. SPEEDSpeed control is important for safety and stability on the slopes. For example, competitive slalom skiers often need to keep their speed high but steady enough so they do not fly out of the run due to an inability to stick to the snow when turning. Even competitive skiers in speed events will need to employ some amount of speed control to be able to properly steer their skis. Air resistance, or the drag force, is reduced by crouching to lessen the projected frontal area (region of body facing the downhill slope). This drag force equation relates drag force to density of the object (ρ), projected frontal area (A), a drag coefficient (CD), and velocity (v): A common misconception is that a heavier person with more mass would ski faster because they have a greater force of gravity (gravitational force equals the product of mass and gravitational acceleration (9.8 m/s2)), and therefore greater downhill acceleration and speed. It is true that heavier people do ski faster, but this is not due to the fact that the heavier person has a larger force of gravity acting on them. In fact, two people skiing the same line in a vacuum chamber would actually reach the bottom at almost the same speed!* The difference in speed can actually be attributed to the fact that frontal area does not increase linearly with weight. Heavier people have a greater force of gravity and a larger drag force acting on them, but that drag force in proportion to the force of gravity on a heavy person is much smaller than a drag force in relation to the force of gravity on a smaller person. So although heavier people might appear less aerodynamic, their weight actually works in their favor speedwise. However, competitive skiers may find extra weight disadvantageous because it makes it harder for them to transfer weight to cleanly parallel turn. Skidding, where skiers slide sideways out of their turn, is a common technique seen in both recreational and competitive skiing to reduce speed. To skid, a skier must turn his or her skis quickly across the fall line—the direction to point the skis for the fastest speed down the hill—so that the force the skier exerts against the hill will overcome the static friction of their ski edges, causing the skier to lose traction with the slope. This leads to a kinetic friction—perpendicular to the fall line—between the skier and the hill that decreases the skier’s kinetic energy moving downhill, lost through scraping snow away from the slope and generating heat. This is the same phenomenon that occurs on ice; when skidding occurs to a certain extent, the skier’s center of mass will be over the snow instead of being over their feet, causing them to fall when their skis slide out far enough from under them. As you can see in the video below, when Lindsey Vonn shifted her body weight too far to the right in a 2007 competition, the force applied by Vonn on her skis was greater than the static friction between her skis and the ground, causing her to skid out of the course into the safety netting. * This slight difference in speed would be because weight affects the force of kinetic friction on the skier. Citations“5 Types of Snow and How They Function.” Snow.Guide, 11 Feb. 2015, snow.guide/snow-types/. “Parallel Turns.” How to Parallel Turn, Mechanics of Sport, www.mechanicsofsport.com/skiing/how_to_ski/parallel_turn.html. “Physics Of Skiing.” Real World Physics Problems, www.real-world-physics-problems.com/physics-of-skiing.html. “Skiing Terms, Snowboarding Slang, Snow Words.” Snowslang.com, snowslang.com/glossary/. Suggested Readings
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