Pendulum Waves

by Deb on February 6, 2012

First watch this video:

How cool is that?

And did you like the setup in the beginning when they showed you there were no tricks, it’s just a line of pendulums of different lengths. This, my friends, is all about maths. And physics, but at this level physics is pretty much applied maths.

Pendulums

Pendulums are incredibly simple to deal with mathematically, because all sorts of things cancel out. It turns out that it doesn’t matter about the weight of the bob or anything else, the only thing that matters in how fast a pendulum swings is the length of the string.

The energy for a pendulum comes from gravity, which is basically the same everywhere on earth so we can ignore it. The bob both powers the pendulum when it swings down, and uses energy when it swings up. So heavy weights make and use lots of energy, light weights make and use less energy. They cancel out. (And before you get excited about perpetual motion, don’t forget friction.)

So to work out how fast it swings all you have to know is how far it is going, which depends on the length of the string. This is why the kid on the short swing always goes faster, even if they are the little one. And why you can go higher but you can’t make it go any faster.

The Video

What this beautiful video is doing is taking advantage of some of the mathematical patterns that relate to the speed and movement of the bobs. Each one is swinging back and forth at a constant rate but sometimes they line up, so that one has swung twice while another has swung 3 or 4 times. That makes them look like they are swinging together, at least for those few seconds, and causes the ‘lines’ that cross each other.

There are also some optical illusions involved as your brain tries to make sense of it, which make them look like they are going in circles rather than back and forth. Or that might just be me.

Another one for the annals of ‘completely useless but really cool’ science. Feel free to tell me about uses in the comments 😉

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{ 1 comment… read it below or add one }

Linda Janesz October 6, 2012 at 1:14 am

Hi Deb,
I loved your video! I’m a science educator who works in a small hands-on discovery area that is part of a public library. I am presenting an afterschool program about pendulums for 8-11 year olds and would love to build this as a demo for them, and to possibly use in our exhibit space afterwards. Could you please give me instructions on how to build it? (Length of strings, size of hexnuts, how you attached the strings, etc.) I would greatly appreciate it!
Thanks, Linda Janesz

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