Why do Skis Slide? (The Real Reason)

by Simon Knott | Updated On: February 18th, 2022
skis-slides

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When you push off downhill on a run it’s easy to imagine the smooth base of the ski simply slides across the surface of the snow. However, there is a lot more going on than you might initially think.

The different forces at play when you ski over snow are very complex. Skiing researchers have identified several effects to explain exactly what’s happening. The force of the ski against the snow is enough to create a thin film of water to slide over. However, powder snow is so light it offers little resistance to the ski and so this thin film of water isn’t created and there is more friction between the ski and the snow. Ski waxes help to reduce friction as wax naturally repels water, reducing friction in the process between the ski and the water.

Your Pressure on the Ski Melts the Snow

Newton’s third law of motion tells us that: ‘For every action, there is an equal and opposite reaction.’  When we apply this to skis on snow it’s easy to understand the force going down through our boots caused by our weight onto the snow. However, it’s more difficult to grasp the idea that the surface of the snow is pushing back with equal force through the skis.

In a separate law of thermodynamics, we discover that the temperature at which water freezes varies with the pressure the water is subjected to. When you stand on snow with skis your body weight exerts enough pressure to reduce the melting temperature of the snow and turn it into a thin layer of water. The friction between the ski and water is far lower than the friction between the ski and snow, hence the skis slide easily over it.

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Thin Layer of Water you Glide Over

As your skis push against the snow, kinetic friction occurs which converts kinetic energy into thermal energy. Kinetic energy comes from the act of pushing yourself down the slope. The thermal energy or heat that’s created, where your skis and the snow make contact also contributes to melting the snow surface. Creating the thin layer of water over which the skis can glide.

As the ski moves forward it is continually melting a new patch of snow to slide onto. This process is repeated so quickly that the skier experiences a smooth and seemingly effortless glide forward. A heavier skier will exert more pressure on the snow surface, melting more snow into a layer of water. This thicker layer of water is needed to overcome the greater friction which a heavier person creates.

The physics surrounding how skis glide over snow is extremely complex, but it has been researched comprehensively.

Photo by Ruth Hartnup licensed under CC BY 2.0

Skiing Can Cause Ice

Once the ski has passed over the thin layer of water the pressure is suddenly released, and the water freezes once more. However, as it has lost its crystal structure it freezes into ice rather than snow. This isn’t apparent on a run because trailing skiers will ski over these icy patches carving them up into snow once more. Sometimes on a hard-packed snow surface, which has been used by lots of skiers, you will see the tracks of ice in among the snow. The preparation of the runs with a snowcat is one of the best ways to break up any tracks of ice.

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Powder and Soft Snow

The flakes of powder and soft snow are very loosely packed with lots of air trapped between the crystals. This light density means there is very little support when you ski over the snow like this, and the skis consequently sink a long way into the snow surface before the snow becomes compressed and can offer support.

The level of support is much lower than when skiing over harder ski surfaces. Consequently, it is difficult to exert adequate pressure to melt the snow under the ski and create the water layer to ski over.

This is the reason why you need more force to ski through powder and soft snow.  In addition, extra friction is also created as your skis and boots sink into the snow. Extra force is needed to drive your skis and boots through the surrounding snow.

Sliding Over Wet Snow

Wet snow is particularly heavy to ski through. The crystal structure of the snowflakes still partially exists, but they are surrounded by melted snow creating a soft slush, which has much more resistance.

In the same way as powdered snow, wet snow is difficult to compress so it offers little support to the ski. At the same time, there is so much water under the ski it cannot release from the water surface and this vacuum effect inhibits the ski from gliding normally.

Skiing in wet snow is always much harder work, as you must physically work against a much denser structure.

Ski Wax

Skiers are keen to reduce the level of friction between the base of their skis and the snow as much as possible to reduce the effort they must put in. The technology of ski materials is constantly evolving but currently, ultra-high-molecular-weight polyethylene (UHMW-PE) is the most popular choice. This layer is very thin, only about 2mm thick. Some polyethylene surfaces are treated to create tiny holes in the surface into which wax can absorb making for a better glide. Similarly, carbon can also be added to the polyethylene to reduce static electricity build-up, which is generated by the friction between the ski and snow surface.

Waxes

Wax and water naturally repel each other, and it is this feature that makes wax such a useful coating for the base of skis. Functionally the wax needs to continually repel the water underneath the ski minimizing friction in the process. At the same time, the wax needs to be durable as it operates in a harsh environment constantly sliding over new snow and ice.

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Types of Wax

There are two types of wax that are used for coating ski bases. The most popular are hydrocarbon waxes, which are usually paraffin-based. These are the most durable waxes partly because their molecular structure allows them to penetrate the polyethylene surface of the ski.

The second type of wax, fluorocarbon, has more functional chemistry. They are manufactured to have negatively charged fluorine atoms attached to the carbon molecules. This negative charge acts to repel water and dirt more efficiently. On the downside, the fluorocarbon waxes are much more expensive, need more preparation, and are less durable. To prepare a ski base a layer of hydrocarbon wax is applied followed by a top layer of fluorocarbon wax. The second type of wax is often used by skiers in downhill races.

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