Another good example thwarted by the truth
Before launching into waves with my first year classes I finished oscillations with a look at some examples of resonance. That means the classic footage of Tacoma Narrows bridge. Just as gallopin' Gertie was getting into her stride one of my students (Jonah) looked up from his computer and announced that Tacoma Narrows bridge isn't an example of resonance at all. Hmm, I thought. Well is was when I was at school. Anyway he sent me the link to the video that he'd googled but I still wasn't convinced. However I did spend some interesting minutes watching some of the minute physics series of which this was one, there are some great little videos here. Further research led (OK via Wikipedia) to the paper that the one minute video is based upon. Resonance, Tocoma Narrows bridge failure and undergraduate physics textbooks (Billah-Scanlan 1990). This has a full and quite convincing explanation. It seems that resonance is strictly speaking when an oscillating system is forced by an external agent that is independent of the system. However in the Tocoma example the movement the oscillating air was being influenced by the movement of the bridge itself making it an example of self excitation rather than resonance. Oh well, I didn't like that music on the video clip anyway.
Tags: Resonance