November 08, 1993

# College Football

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FOOTBALL PHYSICS

John Walters reports from Auburn, Ala. Professor Jackie Sherrill (B.A., Alabama; b.s., frequently), who a few weeks ago theorized that helium-filled footballs produce loftier punts than balls filled with air (SI, Nov. 1), inspired us to create our own models and, using the scientific method, test his proposition. We enlisted the help of Auburn punter Terry Daniel, whose 48.3-yard average, tops in the nation, was the catalyst for Sherrill's theory, and Auburn physicist Stephen Knowlton, a specialist in nuclear fusion.

Hypothesis: A football filled with helium (He), a noble gas with a density less than that of air, will travel farther through space than a football filled with air.

Test: Two balls were inflated to 13 pounds per square inch, one with helium and one with air. Alternating the balls, Daniel punted each one 10 times. The distance and hang time were recorded.

Conditions: 71� Fahrenheit, no wind, sunny skies.

Data: Average distance of air-filled ball was 59.8 yards; average distance of helium-filled ball was 57.7 yards. Average hang time of air-filled ball was 4.93 seconds; average hang time of helium-filled ball was 4.66 seconds.

Observation: "The helium ball feels heavier," said Daniel, "and doesn't take off as well."

Conclusions: Because helium is approximately one eighth as dense as air, a helium-filled football will be slightly lighter than one filled with air. In fact, the mass of the helium-filled ball was measured at 408.4 grams, while the air ball's was 416.3 grams.

"But that's only a two-percent difference," said Knowlton, noting that the difference in the masses of the gases is negligible compared with the mass of the ball. Also, using Newton's second law of motion (F = ma) and a computer code that Knowlton developed to predict the orbits of ions in a fusion plasma, Knowlton discovered that the lighter ball would be more susceptible to the effects of wind resistance.

"Thus," Knowlton said, "giving Jackie his best case—that the initial velocity of the ball is inversely proportional to its weight—helium would still be detrimental to trajectory, because lighter objects, even traveling at the same speed as heavier objects, are more susceptible to the opposing force of wind resistance."

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