Fluttering is usually reserved for fans, flags, and Nicolas Cage's coats during action movies. Under the right conditions, though, it's good for more than just window-dressing. Find out how flutter can be a force of nature....

In life, we can make a few things flutter, namely eyelashes and hearts. Physics flutter pretty much flutters itself. And the greater the flutter is, the more the flutter builds.

Aerodynamic flutter happens when something that can move gets put in a stream that can move it. It can happen in water, or other liquids, but since we live above the sea, despite our recurring dreams of being The Little Mermaid and wearing a shell bikini in a sea full of oil and fish urine, most people see flutter in the air. As air rushes by, for example, a flag, the flag gets jerked one way, then another. The result is continual motion. Pretty enough, but what can it do, besides instill patriotism in today's youth?

Things get interesting when the motion gets more pronounced. Weak flutter damps itself down. Minor motion gets big time resistance, and the result is stillness. As the motion gets larger, though, it becomes ‘self-exciting', apparently because physics wanted to compete with biology for the dirtiest scientific term.

What can a little excitement do? Well, some say that it can knock down a bridge. The Tacoma Narrows Bridge was built in 1940 and soon developed an interesting ability. When the wind got too high, it rolled like the ocean, twisting, turning, and having waves travel from one side to the other. In a move that should make you both intensely grateful for and regretful of modern safety regulation, the powers that be decided that the bridge should stay open. People even took rides over it with their cars on windy days, presumably because they were gearing up for participation in World War II and didn't want to be scared once they joined in.

The bridge didn't last. In under a year the wind got strong enough for the bridge to tear itself to pieces, and flutter may be to blame.

There are many competing theories on how the Tacoma Narrows Bridge finally met its demise, but one of them is aerodynamic flutter. When the wind hit the bridge from below with enough force, the bridge twisted. Since bridges are made of material that doesn't like to twist, eventually enough energy built up to make the bridge snap back down. That energy, in fact, made the bridge overshoot and twist the other way, where the wind caught it and twisted it more. Then it snapped back up again, overshot more, the wind hit it from beneath again and – so on. Every twist and every kick from the wind made the flutter more violent. The energy built up until it ripped the bridge apart.

But flutter isn't just potential for disaster. One company is using it to generate power. The windbelt is a simple structure that can be put up to harvest energy from the air. It consists of a belt strung vertically. On one end there is a couple of magnets stuck between copper coils. As the wind blows and the belt flutters its little heart out, the magnets jiggle back and forth between the coils, which generate electric current. The result is a relatively simple structure that can generate electricity anywhere.

[Via Answers.com, Physics.org and Vibrationdata.com.]