The clip of the wildly swaying bridge deck was actually sped up when it was converted from the original film, some physicists now say.
Seventy-five years ago Saturday, the Tacoma Narrows Bridge swayed and twisted until a section plunged about 190 feet before splashing into Puget Sound.
The collapse of “Galloping Gertie,” as the bridge was nicknamed, proved a cautionary tale in high-school physics classes. Dramatic film shot that day showed the bridge twist nearly 45 degrees before crumbling, which punctuated teachers’ lessons on why physics and the principle of resonance were important.
A new study by researchers at Texas State University, however, claims one famous clip of the action, shot from the bridge deck, speeds up the footage and makes it look more dramatic. The study also highlights other research that shows teachers who blamed resonance had it all wrong.
“We watched that video for literally decades and showed it to dozens of physics classes,” said astrophysicist Don Olson, one of the study’s authors. “The video has been misleading us for years.”
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Here’s what happened, according to Olson, who uses physics to solve mysteries in art and history: Two photographers were filming the bridge on Nov. 7, 1940. One filmed at 16 frames per second from the deck as the bridge twisted in the wind. The other filmed at 24 frames per second from a nearby cliffside as the span toppled into the Sound.
When the film was converted into what would later be used in physics classrooms across the country, both film clips were converted as if they ran at 24 frames per second.
The video of the bridge crashing into the water is accurate; the one of the bridge contorting in the wind, when converted, appears about 50 percent faster than reality, Olson said. It actually twisted more gently than the video shows.
Physics teachers have also been misleading us, according to Olson’s paper, which points to several separate studies about the cause of the bridge’s collapse.
“Ninety-nine percent plus of the physicists reading our article will have been teaching the Tacoma Narrows Bridge as resonance,” Olson said.
Here’s the basic lesson about the phenomenon of resonance: Every object has a natural vibration frequency. When waves vibrate around that object at the same frequency, its vibration becomes amplified. One classic example: When an opera singer hits a high note, the sound waves may vibrate at the right frequency and shatter a glass.
While physics textbooks and teachers have blamed resonance for the bridge’s collapse, they were wrong, the newest studies say.
“The bridge was destroyed by a different phenomenon,” said Bernard Feldman, a professor of physics at the University of Missouri-St. Louis. He wrote one of the papers cited by Olson.
Earlier on the fateful day, resonance caused the bridge to move up and down, but it was actually instability in the air that caused the collapse, Feldman explained. Winds above 40 mph caused air-pressure changes and created vortices that swirled around the bridge, twisting, lifting and dropping it, which caused it to break apart.
Modern bridges receive wind-tunnel tests and are designed wider to handle more air instability, said University of British Columbia physics professor William Unruh, who modeled the collapse with computerized fluid simulations.
He said the misleading video made such an impression on people in the past that the resonance theory sticks.
“They (teachers) showed you the film, and you said, ‘Ooh, that’s impressive … I’ll believe that,’ ” Unruh said. “Unfortunately, physics textbook writers did the same thing.”