Quote:
Originally Posted by jyl
An error in my math - potentially significant.
P pressure in the ideal gas law is absolute pressure. Our pressure gauges read relative pressure, that is, relative to atmospheric pressure of 14.7 psi. So a football for which the gauge reads "12.5 psi" is actually at 27.2 psi absolute ( = 14.7 +12.5). Reduce that by 4% by moving the football from 75 F to 55 F, the pressure goes to 26.11 psi absolute (= 27.2 x 0.96). Subtract atmospheric pressure, and the gauge will read 11.4 psi (= 26.1 - 14.7). So, potentially a full 1.1 psi drop on the pressure gauge (= 12.5 - 11.4). Or a bit less, since the football has some tiny bit of elasticity.
Looks like the Patriots have done some experiments. I assume the NFL has too. Bill Belichick says New England Patriots 'followed every rule' in preparation of footballs - ESPN Boston
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Think you need to redo your math again. Atmospheric pressure is also temp dependent. the percentage applies to both your "constant" and the ball pressure....so you were right the first time.