Re: Why would cooling an object like wood and metal increase its friction?

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. 
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