It is assumed that a smooth surface is slippery while a rough surface is not. However, this is not true with ice. Ice is slippery not due to its being smooth but because its melting point decreases when pressure increases.
One slips on a smoothly polished floor much more easily than on one that isn't polished. So, shouldn't smooth ice be much more slippery than bumpy ice?
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However, contrary to the expectation, a sled goes much more easily over bumpy ice than over smooth ice, which is noticed by people all over the world who have pulled a sled. Ice is slippery not because it's smooth, but because its melting point drops when pressure is increased.
Let's see what happens when we bring a sled or skate. On skates, we bring the whole weight of our body to bear down on a very small area, of but a few square millimeters. Due to this the skater exerts tremendous pressure on the ice. Under strong pressure, ice melts at a lower temperature.
For example, if the temperature of the ice is 5 degrees below zero, and the skaters pressure has lowered the melting point of the ice beneath the skates by 6 or 7 degrees, this ice will melt. It has been theoretically calculated that to lower the melting point of ice by one degree Centigrade, we must exert the rather considerable pressure of 130 kg/cm square.
Will the skater or the sled exert such a pressure? If we distribute the weight of skater (or sled) over the surface of skate blades (or sled runners) we find that the pressure is much less,...
... which shows that there comes in direct contact with ice the far from full surface of the blades or runners.
{In our theoretical calculations we must bear in mind that the process of melting both ice and water are subjected to the same pressure. In the instances described, though, the water, which the ice melted into, is subjected to atmospheric pressure, in which case pressure is required to lower the melting point of ice.}
This gives rise to a thin layer of water between the blades and the ice. No wonder the skater slides, or rather slips, along. And as soon as he moves further, the same thing repeats itself. The skater continually slides over a thin layer of water. It is only ice that has this property. All other bodies are smooth but not slippery.
Coming back to our first point. Why is bumpy ice more slippery than smooth ice? We already know the same weight exerts a stronger pressure when it rests on a smaller area.
When does a man exert more pressure? On smooth ice? Or on bumpy ice? It is quite obvious that he exerts more pressure on bumpy ice because in this case he is supported only by a few bumps of ice.
The greater the pressure exerted, the more readily does the ice melt and consequently, the more slippery does ice become, provided the sled runners are wide enough (this will not apply to the thin skate blades as the energy of motion is expended to slice off the bumps). This pressure-induced lowering of the melting point of ice explains many other things that we see around us. This is why separate lumps of ice freeze into one when strongly pressed together.
Boys throwing snowballs unconsciously avail themselves of this property; the separate snowflakes stick together because the pressure exerted to form the snowball lowers their melting point. To make a snowman, we again apply the principle.
Also under the pressure of many feet walking along the sidewalk snow gradually turns into one solid icy mass.