# Comment on each of the following assertions about skiing. (a) In downhill racing, one wants skis that do not turn easily. (b) In slalom racing, one wants skis that turn easily. (c) Therefore, the rotational inertia of downhill skis should be larger than that of slalom skis. (d) Considering that there is low friction between skis and snow, how does a skier exert torques to turn or stop a turn? (See "The Physics of Ski Turns," by J.I. Shonie and D. L. Mordick, The Physics Teacher; December 1972, p. 491.)

Grade:upto college level

## 1 Answers

Deepak Patra
askIITians Faculty 471 Points
8 years ago
(a)
The downhill racing traverses a larger inclined surface area, and therefore the acceleration of the skis not only depends on the force exerted by the skier but also by the gravitational force. The larger acceleration also accounts for the fact that the skier moves at larger speed and possess larger inertia.
A ski that could turn easily will be a problem for the downhill racing because at larger speeds, the inertia of the skier will also be significantly larger, and any unmeasured turn would put the skier on the wrong track and in a wrong position. Also, to regain its control, a large amount of force should be applied by the skier. Therefore in downhill racing, one would not want skis that turn easily.
(b)
In slalom racing, a smaller track is laid using the gates, and on a less inclined plane. This reduces the acceleration due to the gravitational force on the skier and he/she gets longer time to control the turns. Also, to pass through the gates the skier has to take sharper turns which can only be made if the skis are easier to turn around.
Therefore, in slalom racing, the skis should be made such that they are easier to turn.
(c)
From part (a), and (b), it is clear that the ski must be difficult to turn in downhill racing and easier to turn in slalom racing. Therefore a ski with larger rotational inertia will be helpful and efficient in downhill racing whereas the ski with smaller rotational inertia will be helpful in slalom racing.
(d)
There is low friction between the skis and the snow, and therefore skier use ski poles to provide external force to turn or to stop. Ski poles are pushed through inside the snow, and the grip is used to make turns.

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