|MadSci Network: Physics|
You might want to think things over a bit more, Reid. Are you sure that cooling, by itself, is what increased friction in your experiments? Friction usually involves contaminated surfaces. Contaminants include dust, which can act in several ways, including behaving like small ball bearings. Some contaminants can be very sensitive to moisture. There may be oils, or greases, that can change friction in strange ways. With one surface a metal, oil films that are less than 120 nanometers in thickness will have little effect on friction. This is a very thin oil film. It is equivalent to one drop of oil that is spread evenly over a desktop. Oil films from 200 to 500 nanometers in thickness will increase friction, at least for your low rubbing speeds. With a 250-nanometer oil film, I have measured friction increases of 55%. Above 1000 nanometers (1 micron thick oil film), friction will decrease. Some organic compounds can alter static and low speed sliding friction at even lower concentrations. You indicated that you put your materials in a freezer for three hours. This is a likely source for friction change. Chilled parts will have water condense on their surfaces, unless the room humidity is very low. You also may have had some freezing of this water at the contact surfaces between your blocks and the ramp surface. Water can increase friction. Freezing of parts certainly will increase the apparent static friction. Perhaps you may want to try your experiment again. This time, try warm and dry, warm and wet, cool and dry, cool and wet. Try to have control over your test variables. You may want to find out how much friction change there is from a simple contamination of cleaned surfaces, such as a fingerprint. You may be surprised how different the effect is for different people. Some have more oil, some more acid, some more salt, and some have more transfer of dead skin. All may affect friction. You also may wish to see the effects of different soaps, oils, and waxes. After using them, you may be surprised how difficult it is to get your surfaces cleaned. Run the experiment carefully. You should find that static and low speed sliding friction of wood and plastic against most metal surfaces is not much affected by temperature. The following web sites may give you some more information. Hope this helps. first one second one
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