Falling Slinky Physics - Counter Intuitive

[SnakeyJ]

I've seen various several slo mo videos of falling slinkies, so know this probably isn't new - but this video and discussion of the physics is stunning/excellent. It's rather spooky/counter intuitive, but a great demonstration and interesting talk point for anyone teaching/lecturing physics at any level.

Enjoy - Jake

[JamesF]

I've seen the first clip before, but it's still fascinating.

James

[acey]

Nice video but I'm perplexed by the "explanation". The speaker (Mike Wheatland, Sydney University) manages never once to use the word "tension".

Consider the forces acting on the bottom of the hanging slinky while it is being held. These are gravity (pulling downwards) and tension (pulling upwards). The tension is caused by the weight of the slinky itself, i.e. by gravity. So the two forces balance: the bottom of the slinky is stationary.

When the slinky is released, there is still tension pulling the bottom upwards, and gravity pulling downwards. The bottom of the slinky doesn't feel anything different, so stays still. But the slinky as a whole is falling (its centre of mass is descending, as the guy shows in his simulation). When the top catches up with the bottom there is no tension, no balancing force at the bottom, and the whole thing falls.

We should also consider the top of the slinky. Initially, the forces are gravity (downwards), tension (downwards) and a balancing reaction force (in the person's hand) acting upwards. When he releases the slinky there are only forces of tension plus gravity pulling the top part downwards. When the top part meets the bottom part there is only gravity, and the whole thing falls as a lump.

The complicating factor is that the slinky doesn't just consist of a top and bottom, there's also everything in between, so you really need to consider each bit separately then add them all together. This is what he does in his simulations.

The speaker is obviously a smart guy so I don't know why he never bothered to say this. Maybe he did, and they edited it out, leaving only his reference to "information", which sounds spookier.

Edit: His research is described here:

http://www.physics.u.../~wheat/slinky/

His paper is here:

http://arxiv.org/abs/1208.4629

I had a (very) quick look. In the paper he and his co-author say, "The physical explanation is straightforward: the collapse of tension in the slinky occurs from the top down, and a finite time is required for a wave front to propagate down the slinky communicating the release of the top". The tension in the slinky is not evenly distributed: at the start you've got stretched coils at the top and unstretched ones at the bottom. So modelling it is complicated, and this is what the authors do in their paper. Very nice.