On Saturday, Patriots coach Bill Belichick gave a surreal press conference where he explained how the Patriots footballs could have deflated without any illegal action on his teams’ part.
His explanation wasn’t the clearest thing I’d ever heard, and I said so in a post covering the press conference. In response to my post, Kirk Hackett, a retired physicist who worked for the Air Force emailed me to explain exactly how the Patriots balls could have deflated even when the Colts balls did not.
It basically comes down to this: The Patriots do something to prepare the balls. Their preparation increases the PSI by 1 pound. Over time that PSI would naturally deflate by a pound once the preparation is done. But, the Patriots hand the ball to the refs who artificially let out 1 pound. So, then the ball naturally loses a pound of PSI, then the ball is underinflated.
Make of that what you will!
Hackett emailed me his credentials after I asked for them. He says, “As far as I go, you might be able to google me. I am retired and used to work at the Air Force Research Laboratory in Directed Energy. At one time, I was President of the Directed Energy Professional Society. As far as academic credentials go, my Bachelors with a physics major is from Dartmouth (1977), and my PhD in Physics is from MIT (1983). I could dig my diplomas out of the closet, but that is a bit of a hassle.”
Here is the full email:
As a physicist, and having listened to Belichick’s press conference earlier today, I can tell that what he said made a lot of sense from a scientist’s point of view.
When the football is rubbed and squeezed, work is being done on the ball, and it will get hotter. If you want, describe the heating as due to friction. When the rubbing stops, the ball will cool back to the environmental temperature. The time constant for cooling depends basically on the materials used in the ball, but pigskin is likely a very poor thermal conductor, which would mean longer times, other things being equal.
The air inside the ball, only a few grams in weight, will equilibrate with the walls of the football quite rapidly – seconds.
If the ball is still warmer than the environment when given to the referees, then they will be deflating the ball back to the requested 12.5 psi. By the way, just inflating the ball with a pump adds some heat to it. That is because the air is squeezed until the pressure in the pump is higher than the pressure in the ball before the air goes into the ball. The squeezing heats the air, quite a bit, but the air doesn’t have a lot of mass.
So, now when the ball equilibrates with the environment, by cooling, the pressure will decrease, according to the Ideal Gas Law, particularly if the environment is significantly less warm than the environment where the ball was inflated and then checked by the referees. Obviously, it was a lot colder on the football field, and got even colder later in the evening.
This scenario, in my professional opinion, completely explains the data that has been reported.
Differences with the balls used by the Colts could easily be explained by different preparation procedures, or even longer waits between preparation and the referee check. Especially if the the Colts QB, Luck, likes higher initial ball pressure – as has been reported.
Now, I have something bad to say about the media. You pinheads have been convicting Belichick and Brady without the facts or doing an investigation.
Some of you have even gone so far as to demand the Patriots be disqualified from the Super Bowl. Others have called Brady and/or Belichick a liar.
One of your colleagues at BI, Tony Manfred, is a prime offender.
In a just world, you the media, would be apologizing or get sued for everything you own. This was obviously done with malicious intent. Unfortunately, one of the more unjust aspects of press freedom is that the media is shielded from being sued by public figures. That leaves the media free, except by their own restraint, to lie, sensationalize, and slander. I have been noticing less and less restraint by the media.
At the bottom of this message is a simple calculation on the ball pressure, just moving it from inside to outside that I did a few days ago. It is consistent with the study that the Patriots did, but Belichick’s press conference provided new details that validated my position even further.
Have a great day!
Kirk E. Hackett, PhD
I wrote the following last Wednesday:
There has been a fair amount of bad comment about whether temperature can account for the observed football deflation in the AFC Championship game.
Many of the calculations just use the pressure measured by a gauge on the ball, but this is a mistake. You need to account for the fact that the measurement is a gauge measurement, not an absolute measurement of pressure.
This is the argument:
If the balls were initially inflated to 12.5 psi (the minimum) in a locker room at 75 F, then the pressure inside the balls at 45 F (like at halftime on the field) would be about 11 psi – already down by 2 psi from the nominal 13 psi inflation pressure. The calculation is trivial, uses the Ideal Gas Law, the fact that the pressure measurements are gauge measurements (relative to the base atmospheric pressure, (I used 14.7 psi for the base atmospheric pressure)), not absolute pressure measurements, and assume the football volume doesn’t change.
Additionally, each pressure measurement takes a little air out of the ball, and multiple measurements can also significantly reduce the pressure. Cyclists understand this.
Given the multiple pressure measurements that were made, plus natural leakage due to rough play, plus temperature differences, all this could very easily result in more than a 2 psi pressure drop, measured at or near field conditions.
Here are the numbers:
75 F = 297 Kelvin (Need to use absolute temperatures)
45 F = 280.4 Kelvin
280.4/297 = the absolute pressure ratio = (X +14.7)/(12.5 + 14.7)
Where X is the gauge measurement of the ball pressure at the field temperature, 45 F.
X = 10.98 psi.
2 psi less than the nominal 13 psi. Just from temperature differences.
There have also been reports that the balls weighed less. Ridiculous. Air doesn’t weigh much. You would have to weigh the balls to a couple of tenths of a percentage, and you couldn’t do that by just hefting the balls.
Why don’t you report this and send it along to the NFL, too. It sure would be good to shut up the ESPN sensation mongers.
The NFL should do a real technical analysis, and some experiments.
Kirk Hackett, PhD
Business Insider Emails & Alerts
Site highlights each day to your inbox.