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The Science Behind Deflated Footballs

Could failure to inflate give a team a strategic advantage?

(Sean Justice/Corbis)
smithsonian.com

The culprits at the center of a set of claims against the New England Patriots are not running backs or defensive lineman. They're deflated footballs—11 of them. The NFL is investigating allegations that the Pats used these footballs to clinch the AFC Championship and a spot at Super Bowl XLIX.

So why’s a deflated football such a no-no? First and foremost, it’s against NFL policy—the league’s rules about balls stipulate that players will play with Wilson balls that are provided by the home team and that are inflated to 12.5 to 13.5 pounds.

But the real reason that less-than-filled footballs are controversial is the advantage they give to players, as NPR’s Geoff Brumfiel reports. A deflated ball might have helped players during the game’s rainy conditions by making it easier to grip, catch, and throw—a clear boon in a sport that’s all about the pass. On the other hand, deflated balls could present a real disadvantage, physicist John Eric Goff tells Brumfiel. He explains that underinflated balls have less mass—and less mass means that “the ball can decelerate faster when you throw it.”**

So why didn’t referees notice that the balls were deflated while in play? Some have speculated that cold temperatures caused a slow loss of pressure (the same principle of compression that can cause tires to lose pressure during the winter)—which could account for the difference.*

Whether they were deflated by fair means or foul, one thing is clear: according to the NFL’s internal investigation, 11 out of 12 balls used in Sunday’s championship game were underinflated. Will the reasons—and ramifications—force other teams to up their inflation game? That’s anyone’s guess, but it's safe to bet that Super Bowl parties will be abuzz with inflation speculation.

*Update: This post originally cited a Boston Herald story, which reported that at least one physics professor thought the temperature could not have been the only factor in the balls' deflation. But we've since heard from that professor that there was a miscommunication with the Herald that is being clarified and that though temperature can account for a small change in the ball pressure, anything substantially larger would call for another explanation. So we've updated the post to remove the misreported information and regret the error.

**This paragraph was updated to remove incorrect information about how underinflated balls might benefit both teams; since teams use only their own balls, whatever benefits or drawbacks an underinflated ball would have would accrue only to one team.

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