Lesson 47: Standing Waves and Resonance

In the last section you saw how waves can build each other up or rip each other down (by constructive or destructive interference).

There are a couple of things we can put together to see an interesting effect based on this.


To understand the special type of constructive interference mentioned above, think of the following (and slightly more scientific) example.

In the above example you are making sure that the frequency of your wrist flicks matches the frequency of the wave itself. This is resonance.

According to legend, a large number of Roman legionnaires were moving from one location to another by foot. They had to cross a rather large bridge to get across a large river. As they crossed the bridge, they continued to walk as every good soldier is trained to… in sync. Left, right, left, right, etc. You can imagine that this started some waves shaking through the bridge. Unfortunately for the legionnaires, they were walking in just the right (or I guess from their view, wrong) way that the waves they were creating with their marching were adding onto each other constructively. The amplitude of the waves increased rapidly until the bridge finally collapsed under their feet! Most of them died from the fall or drowned in the water. How's that for a resonant frequency!

Do a search on the internet for the "Tacoma Narrows Bridge" to see an extreme example of modern day resonant frequency. This bridge fell apart because the winds blowing across it matched its natural resonant frequency. You can watch a brief video of "Galloping Gerdy" (as the bridge was called) bouncing around by clicking here. You can also view pictures in my Picture Gallery.

Standing Waves

Standing waves are just an extension of the concept of resonance.

Watch a video of me making standing waves (with a giant spring!) by clicking here. Requires Windows Media Player 9 or later and a broadband connection (dial-up connection not recommended).