Equation: Formula for Calculating a Skycraper's Sway

From Wired Discoveries:

A skyscraper is a giant tuning fork. Give one a good knock — like with an earthquake or a heavy gust of wind — and it'll start vibrating at its own natural resonance frequency (about seven octaves below the lowest notes on a piano). If you're on the top floor of, say, the 1,667-foot-tall Taipei 101, you could find yourself swaying back and forth abruptly, a total of up to 2 feet within five seconds. Highly barfogenic. So Taipei 101's designers hung a pendulum inside the building — in this case, they used an equation like the one below to determine that the megastructure needed a 730-ton weight with giant shock absorbers bolted to its bottom.

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