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http://www.real-world-physics-problems.com/physics-of-skiing.html Advanced skiers turn by sliding the backs of their skis across the snow. Since the fronts of their skis don’t move much, the skis end up pointed in a new direction.

a. To make it easier to slide the skis sideways, the skier “unweights”—reduces the force pressing the skis downward against the snow. The skier does this by jumping upward. How can the downward force that the skier exerts on the snow be less than the skier’s weight?

b. Less skilled skiers sometimes turn without unweighting—they push their skis sideways so hard that the skis slide anyway. This technique is exhausting. Why does it require so much work?

Explanation / Answer

(a) : (If the skier is willing to accelerate downward,) he or she can simply not push downward on the snow so hard.

Why: There is no law that requires the skier to push downward on the snow with a force equal to his or her weight. In fact, if the skier is accelerating up or down, they will not be exerting a force equal to their weight on the snow. Acceleration requires a non-zero net force, so the snow must push on the skier with a force that doesn't balance the skier's weight. If the snow pushes upward extra hard on the skier, the skier will accelerate upward. If the snow pushes upward weakly or not at all, the skier will accelerate downward. The skier can choose the amount of upward force the snow exerts on him or her by choosing how hard to push down on the snow. The snow merely responds with an equal but oppositely directed force--in accordance with Newton's third law.

(b) : Without unweighting, the force of sliding friction is large. Since the work done against sliding friction is the product of the force needed to slide the skis times the distance of the slide, having a large sliding friction force to overcome makes the work required to slide the skis sideways large.

Why: The more force pressing the skis against the snow, the larger the force of sliding friction that opposes the slide. Since the skier must overcome that sliding friction force in doing the work required to turn the skis, increasing the sliding friction force increases the work done.

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