Friction: Cool to Hot
Name: Mehile O.
At which point (in measurements of speed) does air
friction start heating instead of cooling?
Example: If you were to hold a spoon out of a car window, the spoons
temperature would drop, while supersonic jets must have every nut,
bolt, and rivet made out of special alloys because anything made out
of steel or aluminum would turn to putty due to the heat generated by
the friction with the air.
Just something I have wondered for YEARS.
Note: If there is not a specific span or point of conversion due to a
multitude of inherent variables, is there at least range that can be
offer through an educated guess? I just hope to get some sort of
answer before this quandary drives me to the point of madness :)
Curiosity has always gotten the best of me.
When you stick a spoon out of the car window it does not cool unless it
has been heated above room temperature first or it is damp. The air
flowing over the spoon simply removes excess heat (i.e., heat that
results in the temperature being higher than that of the air). If the
spoon is damp you will get a slight evaporative cooling but once the
moisture is gone the temperature will come to the air temperature.
Note that this means that if the spoon is initially cooler than the air
(been eating ice cream again?) it will heat rather than cool when you
stick it out the window.
In summary, at low speeds, the effect of air flow over an object will
depend on the relative temperatures of the air and the spoon.
At all speeds you are doing work on the air as the spoon (or airplane)
flies through it (compressing the air in front of it). Just like
friction with solids, this work is stored as heat energy resulting in
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Update: June 2012