Tag Archives: physics


I threw this together last night to try out an arrangement for a practical demo Elin is planning to build for a new show at the Centre for Life. I thought it might find wider use than just us sitting on the sofa saying “Coo!”. To be fair, it already has found wider use: Rosy from Cambridge Science Centre is staying with us, and was also saying “Coo!”.

One of the things that’s rather hard to wrap your head around when looking at waves in water is that the individual bits of water don’t translate with the wave propagation. Which rather obviously has to be the case once you realise you’ve been staring at the waves coming towards you for quite some time and yet the sea (tides permitting) hasn’t swept you away. Nevertheless, it’s one of those situations where a diagram helps, and an animated diagram helps a lot.

So here’s that animated diagram. For anyone who cares about such things, I built this in Apple Motion: it’s one (rotating) object, replicated with a rotation shift, which makes it very easy to play with different arrangements. It’s particularly interesting when the movement discs overlap. Maybe I’ll build another animation of that…

Anyway, you’re welcome to use the clip, though you’ll want to download and use this high-definition version (3Mb .mov, right-click and ’Save target as…’, and all that). It should loop smoothly enough.

Understanding the chain fountain

Our chum Steve Mould has had a bit of a hit over the last few years with his chain fountain demo:

There have been arm-waving explanations (some more convincing than others), but recently John Biggins and Mark Warner in Cambridge have published a much more complete description of what’s going on.

Now, the video at the head of this post makes me wince a little, partly because I’m a snobby film-maker but also because the section from about 3:10 (about centripetal force) could, I think, be clearer. It’s not explicit, for example, that in the steady state the chain velocity must be the same all along the chain: that’s why the ‘ball being thrown straight upwards’ analogy works. It’s not that the ball is stationary at the top (if the ball’s path is arced like the chain’s, the ball isn’t stationary), it’s that the ball’s speed changes during its flight. The chain can’t do that.

There are several similar conceptual jumps which make sense only once you’ve understood the process, not whilst you’re in the process of understanding – a common mistake in academic scicomms, if I’m being snarky. You can see the result of these skipped steps in the comments thread at YouTube, which pretty much mirrors what I’d expect. That is: people tripping over precisely the parts the film covers poorly.

However, the torque argument for a force component from the beaker is lovely. The macaroni-on-a-string demo isn’t properly convincing as shown, but it does offer the beginnings of something terrific.

So, critical me thinks there’s better scicomms still to be done around this, but what delights me about the whole episode is that ‘proper’ physics is being done from a scicomms curiosity. It’s also a remainder that often science communicators stop just before the science gets properly interesting: we chicken out at the arm-waving stage rather than aiming for the greater satisfaction that comes from deeper understanding.

Bravo, Steve et al. There are precious few stories like this.

The Rutherford School Physics Partnership has a PDF of more chain problems, if you want to roll your sleeves up and get stuck in.

Ingenious, cheap way to investigate Boyle’s Law

I’ve just taught Boyle’s Law to my Year 13s and made use of the standard apparatus for demonstrating how volume changes with pressure… only I didn’t use it to do a demonstration. It was a small class, so I thought I’d try something different: I presented the class with the apparatus, told them nothing about it, and challenged them to have a play with it and a) work out what it did, then b) use it to tell me something interesting about how the world works.

The students told me later that they liked the activity because it “made them think” and they seemed to have enjoyed the process of being free to discuss ideas and work together to solve the problem I had set. I think it was a successful activity (although I suspect some students got more out of it than others), however, I wish I could have had more sets of the equipment so they could have worked in even smaller groups or even individually to explore Boyle’s Law. Next year, I might use this – a cheap, ingenious way to allow students to arrive at Boyle’s Law through experimentation:

UPDATE: Since writing this, Bob Worley has been in touch to tell me of a similar approach from CLEAPSS to allow students to explore Boyle’s Law and Charles Law with guidance available here.

Not The Alom Shaha Motor

One of the great joys in my life is to come across a new science demo, particularly if it’s an elegant, simple one. I can take credit for introducing one of my favourite science communicators, Michael de Podesta, to this demo of the motor effect. Michael kindly calls it the “Alom Shaha Motor” but I can only wish that I came up with this idea myself. Jonathan and I have made a film about this, but here’s Michael’s own, elegant, simple film of the demo.

Eddy currents

Alom and I are filming at the moment, hence things being rather quiet around here. However, the above caught my eye. This demo is typically done with a long length of copper pipe, and the magnet takes many seconds to fall through. It’s effective on a stage.

The tall narrow pipe, however, is precisely the wrong shape to film, and on video the demo doesn’t work so well. This shorter length of fatter pipe, with an appropriate magnet, has more impact on camera.

Same principle, same demo… but different treatments for different audience contexts. So, lessons for us all:

  1. Don’t assume that the way you’ve seen a demo performed is the best way. Always look for improvements.
  2. It’s not just the demo that matters, it’s the way you use it.

That separation of ‘content’ and ‘treatment’ is, for me, an absolutely key concept.

Tip of the hat to my dad for sending this in.

The black holes have arrived!

A ball bearing illustrating the Schwartzchild radius of the Earth.
A ball bearing illustrating the Schwartzchild radius of the Earth.

I’ve ordered a handful of 18mm steel ball bearings for a demo that isn’t a demo, it’s merely a handling object, but handling objects can also have power and impact.

Hold out your hand and feel the weight of this ball on your palm. It’s pretty heavy. The metal is dense. Now imagine the impossible.

Imagine that all of this room is squashed and squeezed into the ball. It’s pretty tightly packed and much more dense. Now imagine the whole building is inside this ball. Hardly conceivable, but we’re not there yet. The stuff we’re made of contains lots of gaps – tiny gaps we can’t see, but gaps nonetheless. If all those gaps were filled in with more stuff we could squeeze even more mass into the ball. The whole city. The country. The entire world.

If the whole Earth, everything in it and everyone on it, were so densely packed that it could fit inside this steel ball, it would be a black hole. So dense that nothing could escape the pull of gravity.

It’s incredible how powerful something can be when brought down to an intimate, handle-able size.

The Earth has a Schwartzchild radius of 9mm. If the Earth was a black hole it would comfortably fit inside my wedding ring. That’s amazing.

Footnote: Please make sure you read Dave Ansell’s comment below. We’re not aware that any black holes the mass of the Earth exist. Black holes with larger mass would not need to be so dense.

Magical Balancing Can

I suspect many of you who watch the above video will know exactly how it’s done but it’s not immediately apparent to everyone, especially if you choose to present it in a way that isn’t quite honest about what’s going on.

I use this demo in my teaching to introduce the idea that an object will topple over if the line of action of its weight lies outside its base. I usually present it as a challenge: I start off with two identical (apparently) empty drink cans on my desk (yes, I know the ones in the video have slightly different designs). I offer one of the cans to a student and challenge him or her to balance it on the edge of the base. I tell them I’ll try to do the same with the other can. I make a big show of concentrating, then reveal that I have managed to make my can balance while the student’s can keeps falling over (this usually gets a gasp of approval – as I think the video shows, the can balanced on its edge looks quite disconcerting). After the initial surprise at my being able to balance the can, the students usually guess that something’s not quite right.

I think this demo works well presented as a “magic trick” because it captures students’ attention and provokes the question “what’s going on?” or “how does that work?” and that’s when the discussion begins…

UPDATE: I’ve had a couple of responses to this post on Twitter and elsewhere. I should perhaps have said that using this type of approach may not be suitable for all teachers – you have to be comfortable with the way you present a demo to a class and if showmanship isn’t your thing there’s no point forcing it (although I’d argue that this particular demo requires very little in the way of showmanship to present as a “magic” trick). We touch on this issue in our forthcoming film Demo: The Movie.

A teacher contacted me saying it was a shame I didn’t provide an explanation as it would make it easier for teachers to do the demo if they knew exactly how to do it. So, here’s the trick: place a little water in the can you want to balance before your lesson. The easiest way to judge the correct amount of water is to hold the can in the balanced position then pour water into the can until you feel it just balances. Alternatively, you could pour in liquid wax and let that set so that you have a pre-prepared can that you can keep in the equipment cupboard.

Pearls in Air / Pearls of Water

This is an example of a demonstration where video doesn’t come close to capturing the awesomeness of seeing it for real. I love seeing students have the same reaction to it as I did when I first saw it – one of joyful wonder at seeing something which appears to defy the laws of physics, of seeing something impossible.

Pearls in air can be a tricky demonstration to set up and I have to confess that, until making this film, I’d never had to set it up myself as I’ve always worked in schools where the physics technician did it for me. The version shown in the video isn’t perfect – it’s possible to get a better looking stream of “pearls”, but I’m OK with that because it’s honest in its depiction of what can be achieved in a limited amount of time with limited resources.

I find this demo incredibly useful for teaching about projectile motion and it’s a nice companion to the monkey and hunter demo which I think was the first demo film Jonathan and I made together.

Get Set Demonstrate logoThis film was produced for the Get Set Demonstrate project. Click through for teaching notes, and take the pledge to perform a demonstration to your students on Demo Day, 20th March 2014.