Sunday Science With Kids
Chain Fountain
Gravity-defying bead chain, a.k.a. Newton’s beads or Mould Effect
We were packing away our Christmas decorations a few weeks ago, and my ten-year-old daughter had an idea to try out a simple experiment with a bead chain that we had seen on a science show on TV. Before going into further details, here’s a video of our experiment.
When you let go of one end of the bead chain out of the container, and let it flow onto the ground, the chain freakishly lifts up into the air and flows over the edge of the container without touching it!
This phenomenon is known by a few names — chain fountain, Newton’s beads, and also Mould Effect! (Not the kind you find in a dirty bathroom.) We found it fairly simple to recreate with a five-meter bead chain and the plastic box it came in. But the physics of this somewhat counterintuitive observation does not appear to be that straightforward.
Simple Explanation with Inertia
The TV show we picked this experiment up from explained it quite simply as ‘due to inertia’. The part of the chain falling down has a mass and moves in the direction of the ground, and hence possesses downward inertia. At the same time, the portion of the chain between the top of the pile inside the container and the edge of the container has to move upwards to clear the edge — therefore has upward inertia. But, since the section of the chain falling down is longer, the downward inertia is higher, and with the chain held in tension, the upward-moving portion tries to match the downward force and therefore lifts way above the edge.
But this theory seems over-simplistic. Doing a bit more digging around the Internet, I found a few videos that showed a chain fountain forming when the end of a chain laid flat on a board is dropped off the edge. There are also observations on how the chain being stacked affects the height of the fountain being formed. Then, there is one video that shows that the bead chain actually falls to the ground much faster than if it was just falling under gravity. These observations indicate it is more complicated than it seems at first.
History
The problem of the chain fountain does not seem to be that old. It is called the Mould Effect because it was brought to the attention of the world by British TV presenter Steve Mould in 2013, even though it seems to have been discussed a few years earlier in various circles including physics competitions. I am not sure where the name Newton’s Beads comes from, but it is probably more due to being a fairly new problem that should be explained using Newtonian Physics (rather than something like quantum physics) instead of something that was observed by Issac Newton himself.
Proper explanations?
Even though John Biggins and Mark Warner of Cambridge University have published findings that explain this effect as due to a certain ‘kick-back’ effect of the coiled chain, the impression I get is, this is still not fully explained by physics. No! I’m not suggesting there is something paranormal going on here — just that it is not explained well enough using physics yet.
The following YouTube video has an interesting discussion on the topic of chain fountains, with a fair bit of maths, which is beyond me these days, and some interesting ideas.
