Sierra Trading Post Explores: Ski Physics

As I stood in the lift line the other day and watched skiers zigging and zagging down the mountain, I found myself trying to remember my basic physics. I tried to work out how an experienced skier might unknowingly use physical forces to their advantage when they pick up speed and rip down a groomer. I was also trying to figure out why I, a cautious skier from the flatlands of the Midwest, instantly want to drag my ski poles and skid through turns when I feel the need to slow down on a steep slope.

Suddenly, more questions started popping into my mind, like "Why do I see US Ski Team members scraping fresh snow off of their practice course?" and "Why in the world are they practicing in what look like skin-tight leotards on a sub-zero day?" and of course, "How can a better knowledge of physics help all levels of skiers when they're shopping for skis and ski gear?"

After a bit of digging, I found all of the answers to my questions in a basic physics equation.

Ski Speed: Maximum Velocity, Minimum Drag


When thinking about alpine skiing from a physics point of view, it all comes down to velocity and drag force. The equation for drag force looks complicated, but the ski lesson we can learn from it is simple. Want to increase your speed on the slopes? Minimize your drag. Want to slow down and take in the sights (or catch your breath)? Increase your drag.

Ski Physics

The drag force (FD) equation is as follows:

FD= ½C?Av²

Where:

C = Drag coefficient

? = Density of air

A = Projected frontal area of skier perpendicular to flow direction (that is, perpendicular to velocity v)

v = Velocity of the skier relative to the air

Check out this website for a more detailed explanation of the drag force equation.

What Creates Drag on the Mountain?


Snow resistance and air resistance are the two main sources of drag for skiers. Frictional resistance on the snow is often caused by the surface area of the skis, and air resistance is caused by the "projected frontal area," or area that's perpendicular to the direction the skier is going. To see physics at work, look at two different types of skiers: the competitive skier and the cautious beginner.

See Physics at Work on the Slopes


The goal of a competitive skier is to decrease drag, or resistance, as much as possible in order to increase velocity. They carve turns on the skis' edges, keep their skis pointed in the direction they want to go, crouch low, and wear tight clothing. Turning on the skis' edges and keeping the skis pointed in the direction they're going decreases frictional snow resistance, while crouching and wearing tight clothing decreases air resistance. The answer to my question about competitive alpine skiers scraping fresh snow off their practice course lies in the factor of snow resistance. These skiers are aiming to go as fast as possible; they don't want loose snow causing resistance and slowing them down.

Decreasing resistance is decreasing the drag force, thereby increasing velocity and getting the skier to the finish line faster.

How skiing physics works

Now, imagine a beginner skier to see drag force at work. They're skidding through turns, creating a "pie slice" with their skis, and standing straight up in a puffy jacket with arms out and poles dragging. All of these actions increase snow and air resistance, thereby increasing drag and decreasing velocity.

How physics affects skiing

Carving vs. Skidding Through Turns


Delving further into the idea of snow resistance and its negative effect on speed, we can now understand exactly why the faster, more skilled skiers carve turns while the slower skiers skid through them. Knowing what we know about snow resistance and drag force, it's easy to understand that carving a turn on the edges of your skis is more efficient because it strictly limits the snow resistance — the speed-slowing friction between the skis and snow — to only the edges of the skis. A perfectly carved turn avoids skidding altogether, and the perfect carver always has his skis pointed in the same direction as his velocity. No wonder perfect carving is a skill that even the most experienced alpine skiers find challenging to execute on every turn!

The next time you watch a US Ski Team slalom competition, keep a close eye on the skier's skis and stance. Can you tell by their stance and turn technique whether or not their time will qualify them for a top spot?

The next time you're hitting the slopes yourself, tweak your ski technique a little with the elements of drag force in mind and see how fast or slow you can go by putting your knowledge of ski physics to work.

Use Your Physics Knowledge for Smart Ski Shopping


Always keep physics in mind when you're shopping for a pair of skis. For instance, racing skis are narrow and feature a directional shape to help with carving turns at high speeds. Want to speed down groomers and carve turns? Lean towards a narrower sidecut (check the specs tab for clear sidecut measurements). Want to get the most out of powder days? Choose "fat" skis with a wide waist and an early-rise rocker that floats in powder (and are not optimized for speed or carving turns on groomers). Remember that a better grasp of physics always leads to better choices when you're ski shopping!

See our helpful Alpine Skiing Guide for more information on all of the different types of skis and what kind of conditions they're best suited for.
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