FRICTION

Measuring the time it takes the cart to travel down the track when the track is tilted by a very small angle, the results are not likely to come out very close to the prediction. The reason is that for such small angles, the effects of “friction” are noticeable even on an air track. In introductory physics texts, the word “friction” is used to cover a multitude of sins. With the air track, there is no physical contact between the cart and track. But there are air currents that support the cart and come out around the edge of the cart. These air currents usually slow the air cart down, giving rise to what we might call friction effects.
In common experience, skaters have as nearly a frictionless surface as we are likely to find. The reason that you experience little friction when skating is not because ice itself is that slippery, but because the ice melts under the blade of the skate and the skater travels along on a fine ribbon of water. The ice melts due to the pressure of the skate against the ice. Ice is a peculiar substance in that it expands when it freezes. And conversely, you can melt it by squeezing it. If, however, the temperature is very low, the ice does not melt at reasonable pressures and is therefore no longer slippery. At temperatures of 40° F below zero, roads on ice in Alaska are as safe to drive on as paved roads. When two solid surfaces touch, the friction between them is caused by an interaction between the atoms in the surfaces. In general, this interaction is not understood. Only recently have computer models shed some light on what happens when clean metal surfaces interact. Most surfaces are quite “dirty” at an atomic scale, contaminated by oxides, grit and whatever. It is unlikely that one will develop a comprehensive theory of friction for real surfaces.

Friction, however, plays too important a role in our lives to be ignored. Remember the first time you tried to skate and did not have a surface with enough friction to support you. To handle friction, a number of empirical rules have been developed. One of the more useful rules is “if it squeaks, oil it”. At a slightly higher level, but not much, are the formulas for friction that appear in introductory physics text books. Our lack of respect for these formulas comes from the experience of trying to verify them in the laboratory. There is some truth to them, but the more accurately one tries to verify them, the worse the results become. With this statement in mind about the friction formulas, we will state them, and provide one example. Hundreds of examples of problems involving friction formulas can be found in other introductory texts.