 So why is it important to build some things that are highly damped? Well, it's not always true that you want things to oscillate. In the 1940s on a particular windy day, there was a suspension bridge at Tacoma Narrows that was really unfortunately driven rather hard by the wind. Now the wind wasn't pulsing, so it wasn't like a child being pushed on a swing, but the wind was pushing and the oscillation of the bridge wasn't sufficiently damped. What that meant was that the oscillations just kept on getting bigger and bigger as they were being pushed by the wind. You can see there's a car on that bridge and there's also a person there. And that person was trying to get to his car because his dog or indeed his daughter's dog was in that car. Eventually he had to give up and go away because that bridge did not survive that oscillation and he only just made it free himself. You can see that the bridge went down. This is a famous example of resonance. It doesn't just hold to things like bridges and whenever you're building a skyscraper, it's really important that you make it so that doesn't have one good frequency that likes to oscillate at. When you're building a skyscraper, there's a natural tendency to make every floor the same and to make them out of the same materials in the same way. And that does in fact tend to lead to very clear resonance frequencies for your skyscraper. And that's bad. So engineers have to very deliberately mix up how they build the interior structures of skyscrapers when they're going higher and higher. So how damped an oscillator is is called its quality factor. If you have a really high quality factor oscillator, then it tends to have a really low amount of damping and will oscillate really hard. Sometimes you want a really high quality factor. Sometimes when like when you're building buildings, you want a very low quality factor. And sometimes you want something in between. If I had a guitar and I made a lovely pure sound, but it wasn't damped the environment, then you'd never hear it. It wouldn't be emitting very much sound. So the fact that there's sound coming off it is how we get our actual music. And yet if it was a very low quality factor, it'd be more like plucking a bench and you wouldn't get a lovely sound. And finally, if I place my finger on this fret here, then actually this note here and this string here are basically the same frequency. And that means that the waves coming off this string, the sound waves, must be the right frequency to try and excite the second one. I'll see if I can zoom in really close there and I'll show if I pluck one string Both of them actually vibrate.