The wobbliest bridge

Here’s a resource for physics teachers: The Tacoma Narrows Bridge collapse in 1940 is captured on film by British Pathé. Winds of only 35 mph set the bridge rippling.

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  1. The Tacoma Narrows bridge collapse is widely studied in engineering schools as a structures problem. The bridge had a natural frequency that just happened to be very near the driving frequency when the wind blew (as it often did around there). The wind acted as positive feedback such that as the bridge began to oscillate, the wind reinforced each oscillation to be a little greater than the one before. Positive feedback is really, really bad in most systems.

    You have to watch for this when you create any structure. Its natural frequency must always be very different from any driving frequency (for example, cars and trucks going over a bridge could push on it in time with its natural frequency).

    Interestingly, Nikola Tesla claimed to have invented a device that had a weight oscillating back and forth at different frequencies. When it detected a frequency harmonic with the building it was in, it would lock onto it and use the positive feedback to shake the building down.