Physics at Its Simplest

From The Wall Street Journal:

Over the last few decades, there have been quite a few science-related books with one-word titles. “Cod” and “Salt” by Mark Kurlansky come to mind, as does Dava Sobel’s “Longitude.” Such books promise their readers a comprehensive exploration focused on a single concept. In the Oxford English Dictionary, the entries for “cod,” “salt” and “longitude” are neither long nor complicated, describing a fish, a mineral and a geographic line, respectively. The books revolve around exactly these things.

But look up “force” in the OED and the entry is significantly longer. Like “time” or “motion,” the word denotes a broad, multifarious concept used in many ways and for many purposes. Henry Petroski’s “Force” explores something broader than even the OED’s mammoth definition anticipates. Mr. Petroski is an emeritus professor of engineering at Duke and a prolific author who often examines a single object—a toothpick, a pencil, a bookshelf—from various viewpoints, sharing fresh and sometimes unusual perspectives. His latest book, using force as an organizing principle, aims to provide a better understanding of what engineers like himself do.

At its core, “Force” is about the everyday physical interactions between people and the material world in which we live. Those looking to better understand arcane scientific concepts, such as Isaac Newton’s “action at a distance” or the nature of gravity and magnetism, should look elsewhere. Mr. Petroski’s book includes only a few scattered formulas and no deep dives into quantum mechanics or Maxwell’s equations. Instead he presents a number of technological vignettes and short histories in precise and meticulous terms.

Arguably no large piece of human-constructed infrastructure lends itself so readily to the analysis of force as the bridge. In 1826 an iron-chain suspension bridge was built across the River Irwell in northern England. The bridge collapsed five years later when 74 soldiers paraded over it. Their unison steps caused the bridge deck to resonate, shake violently and break apart. The disaster demonstrated that bridges and other structures have natural frequencies, and that inducing vibrations of a matching wavelength can have catastrophic consequences. Henceforth, engineers would add or subtract mass to key structural elements to prevent such occurrences.

The engineers who designed the Millennium Bridge across the Thames, which opened in 2000, “made sure that the frequency of up-and-down forces exerted by people walking or running across it in synchrony did not correspond to a natural frequency of the structure,” Mr. Petroski writes. What they did not take into account “was that when we walk we not only push down and backward with our feet—we also push sideways to keep our balance as we shift from one foot to the other.” The relatively puny forces emanating from the tread of pedestrians were enough for Londoners to bestow the nickname “Wobbly Bridge” before the passage was closed, redesigned and reopened.

Mr. Petroski considers the relocation of the Vatican Obelisk, an 83-foot-high monument dating from the 13th century B.C. The obelisk first arrived in Rome during the reign of Caligula. In 1585-86, Pope Sixtus V had it moved roughly two blocks in order to accommodate the construction of St. Peter’s Basilica. Mr. Petroski describes the scene as a complex interplay between forces and the people working with them. “When it was time to lift the obelisk into its new upright position,” he writes, “the engineer demanded—under penalty of death—absolute silence from the crowd of onlookers so that the working crew could hear the leader’s commands.”

It was fortunate that a single voice could be heard. As the day progressed, the ropes used to move the obelisk started to slip and lengthen, endangering the outcome of the project. As the author writes: “Suddenly, a seasoned sailor in the crowd with plenty of experience and feel for rigging ropes and the forces they can bring to bear, shouted, ‘Acqua alle funi,’ which translates from the Italian as ‘water to the ropes.’ The old salt knew that wetting the ropes would cause them to contract, as they did at sea, and so become functional again.” The sailor’s advice saved the day; by dampening the ropes, the workers regained their purchase and the obelisk was successfully moved to the spot where it stands today.

Link to the rest at The Wall Street Journal