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Have you ever wondered how big your feet would need to be to allow you to walk on water?

Okay, maybe you haven't. But Old Dominion University physics professor Lawrence Weinstein has, and his interest in questions such as this is not just academic.

Weinstein is on a mission to teach people how to "estimate." From a column he launched earlier this year in a national physics magazine and from a book that he, together with ODU mathematics professor John Adam, have written, ordinary people can learn how to apply universal laws to everyday questions.

This practice helps us make sense of our world, Weinstein says, and keeps us from jumping to conclusions or formulating opinions that do not conform to universal laws.

The title of Weinstein and Adam's book, forthcoming in April from Princeton University Press, is "Guesstimation: Solving the World's Problems on the Back of a Cocktail Napkin." The title of Weinstein's new column in The Physics Teacher magazine is "Fermi Questions," a term scientists use for the sort of questions that the renowned physicist Enrico Fermi could answer with quick applications of physical laws and numerical reasoning. At the first atomic bomb test, for example, Fermi dropped pieces of paper as the shock wave passed, and from the displacement of the paper and the distance to the explosion he accurately calculated the yield of the bomb.

Questions posed in the column and in the book include estimating the relative amounts of oil used to make our plastic bags and to make the gasoline we burn, estimating the relative accelerations of a person, a Porsche, and a Boeing 747, and estimating the relative dangers of driving to the beach and being bitten by a shark.

Adam touched on estimation in his book, "Mathematics in Nature" (Princeton University Press, 2003), an award-winning account of the mathematics that is evident in rainbows, cloud formations, spider webs, leaf patterns, butterfly wings and many other forms in nature. The book, which is illustrated with Adam's photos, explains how mathematics can be used to formulate and solve puzzles observed in nature. In the process, it teaches the art of estimation and the effects of scale, particularly what happens as things get bigger.

Weinstein is a nuclear physicist who has led atom-smashing experiments at the Thomas Jefferson National Accelerator Facility in Newport News. But he believes that you don't have to be a Fermi to put physics to use. He is among the first of ODU's scientists to volunteer for outreach demonstrations for school children or to teach undergraduates. He says one of the best ways to get your head around physics-and to stay grounded in the physical world-is to apply principles governing matter, energy, space and time to what we see around us.

"I first encountered estimation questions in my high school physics course," he recalls. "I love them because they are a great way of applying physical principles to understand the universe."

ODU physics students also seem to enjoy estimation exercises. Weinstein teaches a seminar course, "Physics on the Back of an Envelope," that, semester after semester, is rated by physics majors as one of their favorite courses.

"One of the things I really like about teaching estimation," he adds, "is that there are frequently many paths to the answer and I am often pleasantly surprised by students who have found a technique that I had never considered. The course also forces students to integrate what they have learned in previous courses. When confronted with an estimation question, they have to apply their knowledge of the world to determine which physical principles apply. They also have to figure out the necessary information not given in the problem."

When a person wonders whether we should use ethanol from corn or gasoline from crude oil, a Fermi Question you could pose would be: How much more energy does your car burn every day than you do? This is Weinstein's answer-"Since your car consumes 5-10 times as much energy daily as you do, even if producing ethanol is 100 percent efficient, and it is nowhere near that, we will need to plow under 5-10 times as much land as we do now to produce that much ethanol. This would have tragic impact on wilderness."

The Fermi Questions column was accepted on a trial basis by The Physics Teacher magazine this spring. The plan was to publish the column over several issues in order to gauge its popularity. But the response to Weinstein's first effort in May was such that the column was elevated immediately to a regular feature of the magazine. In each column, the author poses two questions. The answers are on the magazine's Web site (Visit http://scitation.aip.org/tpt/).

The walking-on-water question posed at the outset is a paraphrase of a question Weinstein asked in his September column. By applying a little physics, we find out that a 200-pound person would need feet with a total perimeter of about 3 miles in order to be supported by the surface tension of water. "Even my son's size 15 feet are not that large," Weinstein writes in the answer posted on the Web site.

This article was posted on: September 17, 2007

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