SSS 2008-12-23

（单词翻译）

This is Scientific American's 60-Second Science. I'm Karen Hopkin. This will just take a minute.

On its opening day, 90,000 tourists traipsed across the London Millennium¹ Bridge. That pitter patter of pedestrian feet caused the structure to visibly rock, earning it the nickname “the wobbly bridge.” Now, in the December 17th issue of the Proceedings² of the Royal Society, civil engineers say they think they know what made the London bridge sway.

It had initially³ been assumed that the movement was caused by a portion of the pedestrians⁴ marching in lock-step. Their synchronized⁵ waddle⁶ could have caused the bridge to oscillate, leading even more of them to tread in tandem⁷. But the engineers say that the same oscillations can be generated by people who are simply plodding⁸ along, just trying to keep their balance—no marching necessary.

Using mathematical modeling, the scientists showed that even walking on level ground people stabilize⁹ themselves by controlling where they place their feet side-to-side. So if they’re walking on a bridge that also wiggles side-to-side, in an effort to keep their balance they may shift their weight even more, making the problem worse. So next time you go to walk across the Thames, remember, the London Millennium Bridge is not falling down. But you still might want to watch your step.

Thanks for the minute for Scientific American's 60-Second Science. I'm Karen Hopkin.