Double Pendulum
A mesmerizing physical example of chaotic motion. Talking about how I got the idea to build a double pendulum and explaining how you can make one yourself. 2021-09-20
Some Backstory
Yesterday, I stumbled upon a video from Make Anything in which he was showcasing a double pendulum he made. Below is a screenshot from said video.
What's awesome about double pendulums is that their movement seems completely random at a first glance, even though they are entirely deterministic. This behavior can be explained by a principle called the butterfly effect.
The butterfly effect, an underlying principle of chaos, describes how a small change in one state of a deterministic nonlinear system can result in large differences in a later state (meaning that there is sensitive dependence on initial conditions).
I vaguely remember having tried to make such a contraption a few years ago, but since I didn't have access to a 3D printer back then, I had to resort to metal rods and hot glue. As you might guess, it didn't work out at all.
However, now that I have way more experience with engineering as a whole, I figured I would give it another go.
How I Built It
After a few design iterations in Fusion 360, I landed on a design I was happy with. Note that the 608 skateboard bearings aren't included in the screenshot below for simplicity's sake.
Once I was done 3D printing all the parts, it was time to assemble them. However, before doing so, it is essential to soak the ball bearings in some isopropyl alcohol to remove any heavy grease from the factory. Then, I let them dry off and I lubricated them with WD-40, a lighter lubricant. Below are some photos of this process.
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I also 3D printed a special part to mount the pendulum directly onto the wall, which worked surprisingly well. After assembling the whole thing, it functioned perfectly! Below is a quick video showcasing the final result.
Final Words
This project is fairly simple, but in my opinion, it is well worth it. Trying to make the pendulum move in the same way twice is realistically impossible, which I think is what makes it so mesmerizing to watch.
The legend says its creator still gives it a nudge from time to time