The recent PopStick Domino Mass Activity at the Science Centre was memorable for several reasons: We had about 300 people weaving popsicle sticks and cheering on the sluggish chain reaction; We probably beat the record for the stick bomb ‘explosion’ with the most human intervention; And we had a very supportive Guest of Honour providing much of that intervention herself.
What amazed me most, though, was the explanation of the physics involved in the popsicle stick domino that was presented before the chain was triggered. With an audience of mostly primary school kids, the expert used terms like structural forces, potential energy and degrees of freedom, and I doubt that even many of the adults in the audience bothered to try and follow that explanation. Which is a shame, since the process is really quite simple!
We all have bent things before – a stick, a ruler, whatever – and know that when we let go, those things usually straighten out again. Same with popsicle sticks: if we bend them in our hands, they push back against our fingers until we let go and they become straight again.
When we weave those popsicle sticks into a pattern like the “Cobra Weave”, we bend them. So they push back. For example, in the illustration above, the green stick is bent slightly around the blue stick, so it pushes down at point F, and up at points G and D. (We are looking down onto the sticks here.) But it’s held in place by the brown and yellow sticks. The yellow stick is pushing down at D, because it’s bent around the green stick and held in place at E and B. And what’s holding the whole thing in place is your finger pushing down on A.
The red stick is pushed down at C and pushed up at B, so when you let go at A, it’s flicked away. With the red stick gone, the yellow stick is no longer held down at B, so the green stick can flick it away. That frees the green stick at D, so it’s flicked away by blue, which then is flicked away by… you get the picture.