Pendulums [4] – Synchronisation

In the previous post, pendulums of the same length (and hence the same natural frequency) oscillated each other. Here, the point is that the pendulums are of slightly different lengths. And yet:

Comprehensive notes can be found in what appears to be the original source of this demo, Bryan Daniels’ senior project at Ohio Wesleyan University.

To my mind this demo isn’t an example of resonance… and that’s a thought we’ll pick up in the next post in this series. Meanwhile, please do share your favourite pendulum demos in the comments. Or, you know, any pendulum-related anecdotes – there’ll never be a better time for those.

Pendulums [3] – Coupled Pendulums

Pendulum Week heats up with… coupled pendulums:

(First embed, doubtless of many to come, from the prolific Brady Haran at Nottingham.)

Look in any of those interminable/popular[delete as applicable] ‘Exciting Fun Science Things to Do on a Rainy Day! Science!‘ books and you’ll likely find this old standby, more commonly done with potatoes rather than creme eggs.

It’s not one of my favourites, partly because I think it needs careful performance to appear as amazing as is usually claimed, but also because the subtlety of explanation required hardly seems worth the effort. This film, for example, doesn’t tell us very much. The explanation bit goes:

“There are little forces as [the connecting string] goes out of line that pull from one to the other, transferring energy from [the first pendulum] … over to that one, and then back again.”

Hmm. All that’s doing is describing what we see and replacing the word ‘swing’ with ‘energy,’ and I’m not a big fan of using ‘energy’ as an arm-wave explanation. Robert Winston’s book, snarkily linked above, explains pendulum movement in terms of gravity and momentum, then adds:

“If two pendulums are attached to the same piece of string, they pass their motion back and forth between each other. One pendulum swings, pulling the string it’s hanging from to and fro. This transfers energy to the second pendulum, which starts swinging itself.”

…which, again, is a reasonable description. Is it an explanation, though? I’m unconvinced.

Neither of these ‘explanations’ has begun to cover why it matters that the pendulums are the same length, let alone pesky details like: the demo still works if the connecting string is perfectly taut, when the driving force is delivered by torsion at the suspension point rather than lateral displacement.

But when you try to write a more satisfying explanation you end up in a bit of a mess. I know I did when I wrote this demo into a children’s TV series back in about 1998. A satisfactory explanation has to include (or at least skirt around) energy exchange, mechanical impedance, and resonant frequency – the sheer amount of physics required is, to my mind, beyond what the demo itself will support.

Better, I think, is this variation:

…which is much more clearly about resonance. The inverted spring pendulums also break the visual connection with the phase demonstration in the previous post in this series, which I think would reduce the risk of confusion were one to attempt linking several of these demos together.

Pendulums [2] – Phase

After the previous post I may have got a little carried away, and we’re declaring this Pendulum Week on Fresh pendulum action every morning.

This beautiful demo wasn’t something I’d seen before this film appeared, though the Harvard demos folks behind it trace its history to the University of Maryland in the early 90s, and from there back to Moscow State University previously. Everything old is new again.

Anyway, it’s a beautiful demonstration of pendulum periodicity and, through that, phase. Note that the previous pendulum demo was about the efficiency of energy conversion, and hence the only real link between these two demos is the pendulum itself. You may spot a theme developing here.

Pendulums [1]

The canonical pendulum demo is this:

It’s so well-known it was included in the recent BBC Challenger dramatisation of Feynman’s last great adventure, previously mentioned on this blog, only to my mind the dramatisation did it badly. With a longer pendulum drop the energy loss is minimal, and you really want your back and particularly the back of your head to be braced against a wall, as shown in the film above. If you stand in open space you’re at significant risk of swaying a little, and with a long enough pendulum you may have only millimetres of leeway.

It’s also one of those demos for which I’ve been wary of using volunteers. If they muck around at all they risk a bowling ball to the face, but sometimes a volunteer’s trust that you the performer wouldn’t let any harm come to them is stronger than their understanding of the physics. This is one of those situations where the science is considerably more reliable than the test subject.

Besides, I reckon the best way of performing this demo wouldn’t involve a person at all, but rather a priceless vase borrowed from a museum. Sadly, I’ve never seen it done that way.


A simple tip for any science communicator, or indeed teacher, intending to perform a demonstration to an audience:

Make sure it works.

That’s it.

It’s such obvious advice it almost goes without saying. Almost, but not quite. Sometimes we become so confident in our performance skills that we forget the basics, and this post is prompted by my having seen two prominent UK science communicators recently do demos which didn’t work.

I’m not going to mention any names and I’ll leave the details vague: this isn’t intended as a personal attack. I’m much more interested in how the demonstration failures felt from where I was sitting at the time – in the audience.

I’m going to try to do a demonstration which never works.”

One of the first bits of advice you’ll receive as a performer is: never apologise. The above line called that to mind, along with the obvious response: “Well, why are you going to do it?”. My hope was that famous science communicator #1 was joking for effect, but he proceeded to spend several minutes of an otherwise fascinating and engaging lecture on an incomprehensible demo involving members of the audience having to stand up and sit down according to a set of instructions neither they nor the rest of the audience seemed to understand.

As best I can tell, the ‘demonstration’ failed to illuminate the bit of science it was meant to illustrate. Certainly, it added nothing to an explanation which had already been provided with a diagram. The lecturer even admitted as much. So why include the demonstration at all?

Perhaps because we’re so enslaved to the notion that we must entertain our audiences with demonstrations that we’ll shoehorn one in if there’d otherwise be too much exposition. Or perhaps the lecture was simply too short otherwise.

From the audience’s perspective, all the demonstration achieved was a measure of audience embarrassment.

Famous science communicator #2 did a demo which simply didn’t work. It was supposed to illustrate that chemicals of a certain family were all good fuels, and it involved separating the wick from the wax of a tea-light. The wick was then squirted with body cream and lit: the body cream would melt, flow up the wick, vaporise and burn.

It didn’t.

The wick caught fire, but it didn’t melt the body cream and it certainly didn’t work as a candle. So, again, an under-prepared demonstration which made the audience uncomfortable, except that communicator #2 proceeded as if it had. The lecture – a team effort – carried on without him, and he distracted me as I watched him continue trying to make it work. He was clearly puzzled as to why it hadn’t.

Prior to doing the demo, science communicator #2 had joked that he had only purchased the cream a few minutes before the lecture, making light of his under preparation. And that was the problem, obviously – he hadn’t checked his demo. He’s a busy man, but I think the audience deserve better. So did the non-geek friend with whom I attended the lecture, who was unimpressed by this demonstration and a number of other shortcomings which we’ll cover in later posts.

Most science communicators are scientists themselves, and we’re accustomed to the idea that science doesn’t always, well… work. Our audiences, however, often aren’t as comfortable with the nuances of statistical repeatability. Every time a demo doesn’t work, we risk the inference that science doesn’t work. Dangerous.

In a classroom situation, however, discussing with your audience (students) why they think the demo didn’t work and – time permitting – trying to fix it can be tremendously instructive.

Demos as political theatre

I’ve finally caught up with the BBC’s Challenger dramatisation of Richard Feyman’s last great adventure, which features this famous and terrific piece of science theatre. It’s sometimes referred to as an ‘experiment,’ which of course it wasn’t, really – the outcome was known and expected, and hence it’s a demonstration.

For our purposes, what it does rather neatly is illustrate the power of showing rather than merely telling, and remind us that such power is not limited to the realm of education. Challenger is plotted somewhere between a tense political stand-off and an engineering whodunnit, with Feynman’s famous O-ring demo as the climax. That a demonstration can serve such a rôle in a movie is something from which we should take heart. Sure, the circumstances were extreme, but if you ever find yourself doubting that demos can be dramatic: well, there’s a demo as the key moment in a drama. Appropriately enough for Feynman: QED.

Challenger is viewable on iPlayer for a few more days, and will doubtless be kicking around on torrent sites for a while longer, or if you’re not in the UK.

Too much information?

In a demonstration lecture we rely on the ‘demonstration’ to drive attention, and shy away from the ‘lecture.’ We run away from aesthetics, emotion and character, which leaves only exposition — and we’re at least dimly aware exposition is the dull bit. Best throw in another explosion.
– via StoryCog – Blog.

Yes, I’m cross-posting to my own site again. But it only looks bad because that laggard Shaha hasn’t written anything here yet. And really, read this, it’s good.

Minimal sequence of action

[cross-posted from the StoryCog blog:]

The sequence here which goes from hard light → hard shadow on scrim/Hitchcock gag → using that scrim to turn the same hard light into a soft source is very nicely thought-out. Sometimes demonstrations are about finding the minimal sequence of operations which makes your point.

Thinking about it, usually demonstrations are about finding the minimal sequence of operations which makes your point.

Banana voltaic pile

via Twitter this morning, chemistry lecturer Mark Lorch:


Potatoes? Yes. Lemons? Absolutely. Not sure I’ve seen a banana pile before, though. Mind you, if you’re going to build a little tower of the things you might as well use acid-soaked paper anyway…

Nice twist on a familiar concept.

ISS Photography

Time was, if you washed up on a desert island and wanted to predict celestial events to avoid being turned into stew, you had to judge things just right so your preferred island happened to lie in the path of a suitable eclipse. Thanks to advances in technology, you can now predict much more frequent and widely-visible, albeit less spectacular, heavenly events. Like visible ISS passes, for example.

I took this photo from the bit of grass opposite my house in northern England, July 2010. It’s a one-minute exposure, and yes, that streak of light has people in it.

Theatre, props and explanations, oh my!