Coffee!! When nerds go nuts, they spend hours studying falling slinkies

FR/N: This exercise with post it notes stacked slows the process down and makes other aspects more evident. We can see here that as the top falls, the force on the bottom holding it up reduces and it begins to fall at first slowly then it picks up as the top catches up with it and initiates a full pancake; I think it may be possible to actually get a config where the bottom begins to move up until it collides with the falling pancake stack part, with a suitably non-linear spring. I am thinking that this is a great physics lab exercise begging to be made into a great student experiment with a computer modelling facet! kairosfocus

News: Pausing in dealing with that crisis. Very interesting. This shows the significance of the perspective that a wave in an elastic material medium is a propagating disturbance, where in effect successive neighbour interactions trigger effects. And so it takes time for the initiating effect, release, to propagate to the bottom of the slinky. And, the first wave that hits is a torsional mode, which is saying that something is happening with the windings along the helical coil as well. And the pancaking collapse -- similar to a building implosion, or to the WTC collapse -- or the near equivalent follows. Here, there is a rotation so it is not a pure pancaking. BTW, notice the tumbling that indicates a degree of rotation imparted by the release then via conservation of angular momentum, a speeding up of rotation rate as the slinky collapses into a smaller and smaller body. Believe it or not, this is also related to the collapse of a giant molecular cloud that then forms a rotating star system. And yes, we can analyse this in terms of a signal that travels with that wave. In effect it takes a finite time for the bottom of the slinky to receive info that something is happening and to react based on the energy that accompanies that info-bearing wave, via the generalised forces and inertial constraints at work. E.g. the first info that arrives triggers a torsional rotation and is followed in the live cases by info related to falling and to rotating. The exercise is also a good study on the integration of real world observations, instrumentation with modern video equipment, and software simulations. It also reveals how sims are not equivalent to reality, e.g. the sim is missing the torsional and rotational effects. That last has relevance to the way we tend to erroneously conflate sim model with reality, a common enough issue in both darwinian evo models and climate change [I bring this last in as NCSE is now also a CC shop]. Not even scientific explanations are to be confused with reality, as the relevant logic of abductive explanation tells us: a false model can make accurate predictions. KF kairosfocus