Like I explained last week. Football Pressure Drop by Temperature The necessary law is called Gay-Lussac’s Law and is fundamental to the study of gases. It was derived in 1808 by Joseph Louis Gay-Lussac. The Kelvin temperature needs to be used to study changes since it is directly tied to the motion of molecules. The Kelvin temperature of a 75 degree Fahrenheit room is 297 K. The Kelvin temperature of a 35 degree Fahrenheit field is 275 K. The basic gas law will use Pf to mean the final pressure as read on the field. Pi means the pressure as read indoors. Tf mean the temperature as read on the field. Ti means the temperature as read inside. P means pressure, T temperature, f means final state, and f means initial state. The mathematical form of the gas law is as follows: Pf = Pi x (Tf/Ti) The ratio of the temperatures is 0.926. That is easy but the pressure to plug in is more difficult. The NFL stipulates an internal pressure of at minimum 12.5 psi (pounds per square inch). That however is the gauge pressure reading. There is normal atmospheric pressure both inside and outside a ball before inflation. This normal pressure is 14.7 psi. Inflation merely increasing the pressure inside beyond the normal. At the start of inflation the pressure difference is 0 between inside and outside. To inflate to the NFL minimum of 12.5 psi means the gauge might read 12.5 psi but the true pressure inside is this plus atmospheric pressure of 14.7. Thus at minimum there is 27.2 psi inside. Since there is 14.7 outside the ball due to the atmosphere, the pressure gauge reads the difference of the two so 12.5. Physics is based upon what is really there so the Pi to use is 27.2 psi. Use the equation now and you will find that the Pf on the field is 25.2 psi. Now stick a pressure gauge inside the ball on the field. The gauge reads the difference between ball pressure and atmospheric pressure. So 25.2 minus 14.7 or 10.5 psi. This is what the NFL is reporting as the difference. They stated a pressure drop of 2.0 psi. The calculations should a drop from 12.5 to 10.5 psi due to the temperature drop. Thus there is no scandal but rather basic physics at work.
Posted on: Sun, 25 Jan 2015 02:26:04 +0000
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