Chain reaction forces

Here’s a weird little phenomenon that should appeal to all of us who think we understand Newtonian mechanics…

First, have a look at this 36-second Youtube video from the excellent Steve Mould:

Now try to work out what on earth is going on… Bear in mind that this behaviour doesn’t rely crucially on the beads along the chain, and you can even get the same effect with rope:

If you’re puzzled, you’re not the only one. Steve Mould drew attention to the phenomenon in a blog post back in the summer (it also contains a link to a really neat slow-motion video), which in turn produced some discussion on Reddit (and some follow-up experiments).

Most recently, an analysis by two professional physicists, Prof. Mark Warner and Dr John Biggins, has appeared in the journal Proceedings of the Royal Society A. [You should be able to read the full article if you’re on a Strathclyde computer; if not then you might be interested in the podcast version.] If their analysis is correct, then all you need to know in order to understand the chain fountain is some school-level physics (tension, gravity, and a little bit of moments) — and a willingness to think carefully in some slightly non-intuitive directions. (Spoiler: it turns out that the really crucial detail is the reaction force that the jar exerts on the chain as it’s picked up. But you’d already guessed that… right?)

(DP)