In this diagram I highlight two forces involved in skiing. The force of gravity and the force normal (the force pushing back by the surface of the slope). However, this is an idealized situation and there are other forces that are assumed away. Why is it safe to assume them away and what forces are they?
Skiing (and most any other human activity) is a complicated affair with lots of different factors working to make an experience. In order to understand how those factors form the experience it is almost always best to start with an idealized situation and slowly work in the additional factors. The old joke is that a professor in an ag. department made a major breakthrough in food science. He was presenting his paper:
Welcome, my paper on ending world hunger is revolutionary so let me being with the presentation of that paper! First, we assume a perfectly spherical cow…
You see, even the scientists and researchers know how silly assumptions may seem, still, they are useful.
Forces Assumed Away
The forces I hand-waved out of the free body diagram above are similar in nature and are wind resistance and friction. Let me pull up the force breakdown diagram:
friction and wind resistance will generally work to oppose the force component b since we are usually traveling in the direction of force b (of course, there is another assumption here and that is we are skiing or boarding straight down the fall line).
My recollection is that frictional force varies only between starting friction and rolling friction. That is, the force of friction has one value when attempting to start in motion and another when actually in motion. I do know for sure though, that the force due to wind resistance is proportional to the speed of the skier or the ‘boarder in the air. This means the faster the apparent speed of a skier into the wind the greater the force of wind resistance. Yes, wind resistance can boost speed as well as retard it, but normally wind resistance works against skiers.