VOX IN THE BOX

O OMNISCIENT Vox,

What enables skis to slide over snow? Is it simply the slipperiness of the "p-tex" base of the skis or does the pressure of skis on soow melt a thin layer of snow to create a lubricating layer of water on which the ski glides? .

BRUCE CHAFEE'86

DEAR MR.CHAFEE.

Given the wide gray of waxes, bases and tuning paraphernalia promising-skiers enhanced speed, one could understandably mistake friction as the skier's greatest foe. Far from it. says Thayer School professor and friction expert Francis E. Kennedy.

According to Kennedy, it is friction that enables skis to glide over the snow in she first place. The friction coefficient between a ski's base and snow is typically .6. As you may recall from physics class, friction—the loss of energy through the resistance of one force by another-raises Contact temperature. But where snow or ice is concerned, a melt-water layer forms and acts as a lubricant, lowering friction and allowing skiers to speed up.

Between the two primary types of ski waxes—kick and glide-we see an application of this frictional irony. Kick waxes, usually ally applied at the ski's front, back and edges, increase friction at low velocities (at kick-off, for example), men the snow shears and the melt-water layer forms, the friction coefficient drops to a value that surpasses even that of an unwaxed base. Meanwhile, glide waxes, usually rubbed into the flat of the ski base, are designed to lower friction at high velocities. Thus. the simultaneous combination of wax and friction makes your ride a swifter one.

Of course other variables contribute to the melting of snow. For instance, the bases of most skis are made of polyethylene, which has low thermal conductivity so that heat remains on the surface, where it can melt snow. Finally, the load placed on the skis (otherwise known as a skier) introduces pressure, which also lowers the melting point of snow.

Vox

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Wax and friction combine for a swift ride