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J. Perelman
"Entertaining physics". Book 1.
Chapter 6. Thermal phenomena

WHY IS ICE SLIPPERY?

On smooth ntercom the floor easier to slip than ordinary. It would seem that the same thing should happen on the ice, i.e., smooth ice should be more slippery than ice lumpy, rough.

But if you happen to carry a loaded hand sled through rough, uneven ice surface, you could see that, contrary to expectations, the sledge slid across such a surface is noticeably lighter than smooth. Rough ice is more slippery than smooth! This is because the slipperiness of the ice depends mainly not from smoothness, and from a very special reason: the fact that the melting point of ice decreases with increasing pressure.

Let us consider what happens when we ride in a sleigh or skating. Standing on skates, we rely on a very small area, only a few square millimeters. And this small area is entirely pressed by the weight of our body. If you recall was said in Chapter two about pressure, you will realize that the skater exerts on the ice with considerable force. Under high pressure ice melts at low temperature, for example, if the ice has a temperature of - 5°, and the pressure skates lowered the melting point of ice, trampled by horses, by more than 5°, then these parts of the ice will melt [Theoretically possible to calculate that for lowering the melting point of ice at 1° requires significant pressure to 130 kg per square centimeter. Whether or not a sledge or a skater is enormous pressure on the ice? If you distribute the weight of the sled (or skater) on the surface of the runners (or skates), you get a number much smaller. This proves that to ice adheres closely not the entire surface of the skid, but only a small part of it]. What would it be? Now between runners skates and ice is a thin layer of water - it is not surprising that the skater glides. And as soon as he moves his legs in another place, there would be the same thing. Everywhere under the feet of the skater, the ice turns into a thin layer of water. Such properties of all existing bodies has only ice; one Soviet physicist called him “the only slippery body in nature”. Other body smooth, but not slippery.

[In the theoretical calculation, it is assumed that during melting and ice and water are under the same pressure. The author also describes the examples, when the water formed by melting, is at atmospheric pressure. In this case, it requires less pressure for lowering the melting point of ice. - Approx. edit ]

We can now return to the question of how smooth or rough the ice more slippery. We know that the same load pressure is stronger than the smaller area it is based. In any case, the man has to support more pressure when he stands on smooth or rough ice? It is clear that in the second case: as it is based only on a few bumps and lumps rough surface. And the more pressure on the ice, the more sufficiently melting and, consequently, ice is more slippery (unless the snake is quite wide; narrow skid skates cutting into the bumps, it's the shape - motion energy is spent here on srezanie tubercles).

By lowering the melting point of ice under significant pressure due to many other phenomena of everyday life. Due to this peculiarity of ice separate pieces it freeze together, together, when they are very pinch. The boy, clutching his clods of snow when playing in the snow, unconscious is precisely this property of ice grains (snowflakes) to smartarse under intense pressure, lowering the temperature of their melting. Rolling snowball for “the snow women”, we again use the specified feature of the ice, snowflakes in the places of contact, at the bottom of coma, freeze together under the weight pressing on them mass. You understand now, of course, why in the bitter cold snow forms scattered snowballs, and “Baba” poorly molded. Under the pressure of the feet of the passers-by the snow on the sidewalks gradually compacted into ice: snowflakes freeze together in a continuous seam.

Entertaining physics J. Perelman

 




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