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Name: Murat
Status: educator
Grade: 9-12
Location: NY
Country: USA
Date: Winter 2012-2013


Question:
Does the air friction depend on the air temperature in a confined space at a given air pressure setting? If I have a confined room at 28 degrees and drop a bouncy ball from the ceiling, would it take longer for the ball to come to a complete stop if I did the same at 125 degrees room temperature? Let us assume I have a bouncy ball with very small drag coefficient.



Replies:
Hi Murat,

Thanks for the question. Yes, the air friction does depend on the air temperature, if the air pressure is constant. The reason for this dependence is the fact that the density of air changes with temperature. The air resistance is proportional to the density of the fluid that the object moves through. Now, the density of air is proportional to the reciprocal of the temperature. So, to answer your question, if the air temperature increase from 28 degrees to 125 degrees, the air friction decreases by a factor of (125+373)/(28+373). I am sorry that I cannot type equations easily in email. Please note the "373" values--they are needed as the temperatures must be in Kelvin.

I hope this helps. Please let me know if you have any additional questions. Thanks Jeff Grell


Hi Murat,

There is a fundamental problem with how I interpret your question. It is not possible to maintain a constant air pressure in a confined (sealed) space, when temperature is varied. When the air in a confined space is heated, its pressure rises.

Air friction (i.e. drag) is related to air density. Heating an unconfined body of air, will reduce its density, thus reducing drag. But heating a confined mass of air will increase its pressure, but its density remains the same (after all, the number of molecules has not changed at all) and thus this will result in essentially no change in drag.

Regards, Bob Wilson



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