Pendulums [5] – Not Resonance

As Pendulum Week continues here you’ll have noticed a pattern building up: that pendulums crop up in all sorts of demonstrations, but it’s often rather tricky to pin down satisfying explanations for their behaviour. Pendulums appear simple and straightforward to grasp, which is usually a good sign for demonstration tools as we want audiences to engage with ideas or behaviour and not be distracted by unfamiliar apparatus. However, I wonder if it’s possible that pendulums are too simple, in that their apparent simplicity seems to lull us into forgetting their subtleties.

Heck, unless you’re in that sin θ ≈ θ small-amplitude space you haven’t even, technically, got simple harmonic motion. Most of the time, pendulums don’t even swing like, well, pendulums. Ouch.

It feels like it ought to be possible to link pendulum demonstrations together in a neat story. A mass on the end of a string is about as simple as physics apparatus gets, surely there’s a delightful sequence of demos which can build successively, one on the other, to arrive at something complex and surprising and revealing about the world? That’s got to be possible, right?

Perhaps it is, but the origin of this series of posts lay in my noticing that pendulum demos aren’t alike, and the distinctions seem to me to be of the subtle-and-confusing kind rather than the subtle-but-illuminating kind.

Probably the best attempt I’ve seen to navigate the resulting swamp was by my colleague Marty Jopson, who made this film for the first series of Science Shack (skip to 2:40 for the start of the show):

Marty and I were co-producers on the series, and if I remember correctly he won awards for this show. I wasn’t, I should say, much involved with this episode (harrumph), but it’s still worth a watch. It gets into some of the subtleties about resonance and synchronisation that we’ve seen in this series of posts.

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