I want to be able to model one stick of a nunchuk being moved about freely via user control, and the other stick reacting/swinging in a way that's believable. I'm not even sure what to google; running into lots of talk about martial arts in general... and the Nintendo Wii.

Could it be modelled with a single pendulum? Single pendulum and a joint maybe? These are things I'd have to learn, and ultimately the goal is something that looks clean and feels convincing "enough", so any tips are welcome. Including suggestions on how to smoothen things out should pure equations result in jittery results.

Many thanks

I'm assuming you're talking about 3D; you're basically asking about physics-based animation.  You might want to look at this:

http://chrishecker.com/How_to_Simulate_a_Ponytail

It's not quite nunchuks, but it will probably get you started.  Also, not gonna lie... I haven't actually used that article to make anything... :-D But it looks like it's on point for what I gather you're trying to do.

Looks really good, thanks.