Because skis are so slippery fore and aft, balancing on them in the sagittal plane is much different than most other situations. You’re in balance fore and aft when the balance axis, not the line of gravity, goes through your feet. This is illustrated by the difference in the way beginners and experienced skiers get off a chair lift that has an unloading ramp they must ski down.
Most people taking up skiing are unaccustomed to standing on things as slippery as skis. Their bodies have learned to balance against the considerable friction that’s been under their feet all their lives, and this is why so many beginners fall over the first time they get off a chair lift. Seeing that they are about to stand up on a slope (the unloading ramp), they assume a stance that anticipates a friction force pushing back up the slope on them. When that friction fails to materialize, they’re left off balance.
The beginning skier’s expectation is shown in figure 5.9a; the reality is shown in figures 5.9b and 5.9c. For this person on a slope, the force that gravity exerts on her can be resolved into two components: Gn, which is perpendicular to the bottom of her skis and is therefore aligned with her balance axis, and Gs, which is trying to make her slide down the hill. In figure a, the surface on which she’s standing can counterbalance both those components. Because the surface is firm, it exerts Sn on her, and because there is friction between it and her shoes, it exerts Sf on her. On skis, depicted in figures b and c, things are different. Sf is, for all intents and purposes, gone, and S can only oppose Gn. Because Gs is unopposed, it accelerates the skier down the hill. In figure b, the skier is in the same stance relative to her feet that she was in figure a. If the skier could hold herself in a position with her balance axis behind her feet, using the tail for support, she might not fall over, but the human body isn’t built in a way that makes this possible. Consequently, the force from the snow, S, stays under her feet and is not aligned with the force it is opposing, Gn. A torque is exerted on the skier, and she falls over backward. In figure c, the skier has aligned the force from the snow and Gn, her balance axis, and although she still accelerates down the hill, she’s stable.