Friction: Cool to Hot ```Name: Mehile O. Status: student Age: 20s Location: N/A Country: N/A Date: 2001-2002 ``` Question: 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. Thanks-a-bunch, Mo 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. Replies: Mo, 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 increased temperatures. Greg Bradburn Click here to return to the Physics Archives

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