A lot of Americans know this footage very well. It's the collapse of the bridge at Tacoma Narrows, on Nov 7th, 1940.
What is little realised however, is that the phenomenon is well known to scientists.
It is called "Resonance" and is best described by saying that something is vibrating in a way it has always wanted to.
One of the best examples is on a bus, standing at traffic lights. The engine idles, and suddenly parts of the tubular structure inside go mad with vibration. Not all, only some. This is also resonance, the engine is vibrating at exactly the same rate that these parts want to anyway, so they go along with it. A bit like when you hear a catchy tune, you cannot help but tap your feet.
Everything has this property, and it is called "natural frequency".
The gales shown in the video above came along and rattled the bridge at just the rate it wanted to. In the end it was too much strain.
Here we come to the "A-Ha!" insight - take an ordinary milk bottle and blow into it. nothing usually happens, but tilt it and blow across the top. If you blow correctly and have the right angle, a pleasing , low, note is heard. This again is resonance. The air in the bottle is rattling at just the rate it wants to anyway because you are making it.
In the video, the Narrows are the bottle, the gale is your blowing, and the bridge, the air inside the bottle. Given the right conditions, the bridge sings a note! Only this note is so powerful it shatters the bridge.
This is an example of engineers not doing their physics homework.
See how many more examples you can find!
Friday, February 29, 2008
A Resonant Discord
Subscribe to:
Post Comments (Atom)
4 comments:
Although this is indeed a very famous video and incident, which did lead to changes in the way bridge engineers design bridges--especially suspension bridges, as I recall, I have forgotten if this event was considered inevitable or not.
This leads me to be curious as to why you seem to be making a blanket statement that this was "an example of engineers not doing their physics homework." Was it obvious this had to happen or was this a unique confluence of events that made this incident possible? I know it is often easy after the fact to assess blame, but was it that obvious before the event that destroyed the bridge?
I can look up this event, but many who come here may take you at your word and not bother to research this famous destruction of the Tacoma Narrows bridge, so I thought I would ask. Thanks for posting this unique bit of history. I thought with the various engineering incidents over the years, this one had simply become lost to posterity. Glad to see it is not lost afterall.
Wyman, you should know me better than that. My comment was merely a flippant one. The incident happened due to resonance, but as to whether or not the engineers should have forseen it, no-one can say.
All that can be said is that there was a risk of resonance and given the research at the time, the engineers did all that was possible with the resources available to prevent just an incident such as this from happening.
All it needed was a freak gale to give us spectacular footage.
Why is it always the engineers who get the blame?
What about physicists doing some more engineering homework?
Maybe then the world would actually see some of the results of physics, rather than so many theories being forever talked about.
But you CAN see the see the results of physics. Without Newton's Laws you would have no space travel, without lasers and quantum physics there would be no supermarket scanners, microchips, tv, computers, without relativity there would be no research to take us into quantum physics, the road goes ever on.....
Theories are only unproved laws, and those just given are the laws that govern all the stuff taken for granted around us today.
But usually it is us who do the reasearch, but the engineers make the money from it!
Post a Comment