Resonant Frequency

This is seriously my favorite topic in all of science! "Resonance" means something fits perfectly, like when a song or an art piece resonates with you and you remember it because of its power. In physics, it's the same idea: resonant frequency is the right-sized wave that fits perfectly inside its medium, and when it occurs very strange things happen.

 Although this is actually a trick of the shutter-speed of the camera vs. the vibration of the string

When you tune a guitar, you are changing the density and tightness of the string until the wave created when you pluck it fits perfectly between the two ends. When the wave doesn't fit, your ear can tell the difference: the sound is just off and doesn't last nearly as long. That's because when a wave fits, it forms a standing wave:

Notice that the wave is only obvious when he's perfectly lined up with the proper frequency

 Resonant frequency once destroyed a bridge when the British soldiers crossing it accidentally hit the perfect frequency of the bridge itself, and it shook to pieces; British soldiers now have a policy to walk off-step on bridges instead of staying in sync! This also happened in Korea, a group exercise randomly hit the resonant frequency of an entire building, causing it to evacuate for fears of an earthquake.

The most famous case, though, is this, the Tacoma Narrows bridge in Washington that collapsed in 1940:

 Might look lame by today's CGI standards, but this is very real

I just found out though that it was NOT destroyed by resonant frequency - apparently physics textbooks have been explaining this incorrectly ever since it happened. The actual explanation is too intense for this space, but it's still a very cool video - and resonant frequency contributed to this bridge's demise, which only shook like this when the wind was exactly 42 mph.