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You've probably seen footage of the Tacoma narrows bridge collapse in 1940 (if not, it's
all over the internet).
And while I've heard that in Seattle they just call it bad bridge building, most teachers
and physics textbooks have probably told you that the bridge collapsed because of resonance
with the wind.
But they're wrong!
That's because a resonance is a vibration driven by a source oscillating at a similar
frequency - like how a swing at the park goes higher and higher as you push it, or the electrons
in an antenna move in tandem with the electromagnetic waves of a radio signal, or how a guitar string
vibrates or wine glass shatters if you sing at just the right pitch.
And that's not what happened at the Tacoma narrows, because people at the bridge that
day measured the windspeed as being fairly constant, or, 'not-oscillating.'
So there was no forced resonance.
It's also tempting to think that the bridge collapsed because of vortex shedding, a cool-looking
fluid process where vortices take turns sloughing off of the edge of an object - but that's
not right either.
What did cause the bridge to collapse is a phenomenon called aeroelastic flutter – the
same thing that causes a strip of paper or blade of grass to vibrate if you hold it tight
and blow straight at the edge.
Basically, with enough wind speed, the bridge started to twist a little bit - but then gravity
and tension in the bridge caused it to twist back down, except too far!
So now the wind could push it in the opposite direction, causing it to twist more violently.
And so on and so forth until, well… the bridge fluttered itself into destruction!
And aeroelastic flutter isn't just limited to bridges - it can cause airplane wings to
hum annoyingly, or vibrate so vigorously as to disintegrate.
But don't worry - the last time this happened on a commercial airplane was in the early
1960s.
You should probably be more concerned with the haunted swingset in Firmat, Argentina
that can swing on its own for days - oh wait, no, that's just aeroelastic flutter.
Sorry ghosts - physics just blew you away.