This is an amazing phenomenon, which you can view by clicking play on the video
There is also a hilarious end to the construction of the model, which I’ll get to a bit later.
First, the amazing thing the clip shows is that if you hold a slinky by its top end and wait until the whole spring is fully stretched and still, then release the top, the bottom of the slinky remains perfectly still, suspended in mid-air, until the top of the spring has fallen such that the slinky is fully closed up, after which the whole slinky falls together to the floor.
I wanted to try this out, so bought a large slinky and did so. While the effect was indeed seen, it all happened so fast that it was a bit difficult to catch on film, even using slow motion filming.
So, I had a brilliant idea of filling a large tube with oil and conducting the experiment with the slinky falling through the oil instead of air. The aim was to slow down the action so that it would be clearly visible, hopefully just with the naked eye. While it did work, and was considerably slower than in air, the whole event was still pretty quick, but with slow motion filming was now very easy to observe over a few seconds. This is the clip at the head of this page.
The maths of this is pretty involved, but can be summarised by thinking of the slinky moving as a wave, and the wave does not reach the bottom of the slinky, ‘telling’ it to move, until the top has collapsed onto it.
The funny end to the story is that while Cameron Holmes and I were repositioning the tube of oil into a sink in a Physics room so pupils would be able to repeat the experiment, Cameron lifted the tube sharply not realising that the rubber bung at the base had got a bit of oil on the bottom and stuck to the bench. The tube went up, the bung remained stuck to the bench and seven litres of oil splurged all over Cameron while I leaped backwards and avoided much of it! Unfortunately this was not captured on film!
The event seemed a bit less amusing once we realised that seven litres of oil made a huge mess of a lot of carpet tiles and needed a significant amount of time to clean up - and eventually some new tiles!
I hope you’ll agree that the film of the effect is worth the suffering!