WIRED | 9/12/2017 | Rhett Allain
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I am going to make a prediction. As people start to get bored with their fidget spinners, they are going to start playing with these double pendulum fidget spinners. The normal spinner has a bearing in the center of some object such that you can hold it and spin it—moderately cool, I'll admit. But the double pendulum spinner has two bearings with two moveable arms. Here's how that might look:

In this case, you hold one of the bearings and then let the two arms move about in a fun and entertaining fashion. Here's a description of how you could make one of these double pendulum fidget spinners yourself.

Physics - Play - Things - Pendulums

Besides just being entertaining, there is some serious physics at play here. Let me go over some of the coolest things about double pendulums.

A double pendulum has two degrees of freedom. That means that with two variables, you could describe the orientation of the whole device. Typically we use two angles—θ1 and θ2 as shown in this diagram (assuming constant length strings).

Angles - Position - Motion - Pendulum—but - Things

You might think that with just these two angles to determine the position it might be fairly straightforward to model the motion of this double pendulum—but no. There are really two things that make this problem difficult. First, the two strings exert forces on the two masses, but these string forces are non-constant: They change in both direction and magnitude. You can't just use some equation to calculate these forces because they are forces of constraint, meaning they exert whatever is needed to keep the object in a particular path. For mass 1, it must stay a certain distance from the top pivot point.

The second problem is with the lower angle (θ2). This angle is measured from a vertical line but this variable by itself does not give the whole motion of...
(Excerpt) Read more at: WIRED
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