The video gaming blog Kotaku has recently released a review of video game physics problems, one of which deals with the amount of hay needed to save a person falling from various buildings. They compare the haystack stopping power to that of a bungee cord, wherein the hay slows the person gradually and increasingly, and can be analyzed using a sort of "spring constant" for hay (this raises its own problems of what an 'average' stack consists of, including density and type). I played a minor role in the creation of the article, and it was agreed that a more accurate model would include the fact that the hay would be compressed under its own weight, so there would be a plateau at which no amount of hay could save you. However, the simpler model was that of the bungee cord, and so they went with that. I have included some graphs that better show what I mean (http://img520.imageshack.us/img520/3331/haystackgraphs.png). The 'scalable' models are the ones used in the post. I would like the limits of the 'limited' model to be tested.
Here is the link to the original blog post: http://kotaku.com/5296222/kota...ut-physics-also-beer

Never jumped in a barn?

The bigger the haystack, the bigger the fall it can take..

But there is a point at which it won't help. It works on a logarithmic scale. Read the article, then read the comments on the article.
Here:
http://www.youtube.com/watch?v=2VRGMGJLqPs

For clarity: the myth is that a sufficiently high hay pile will only help to a certain extent (in slowing someone to 0m/s. People can survive in some cases with or without the hay). The question is, at what point will the hay no longer help to prevent injuries/slow the fall? There is a point at which the stack might as well be concrete. What is it?

One more tidbit: what I meant from "might as well be concrete" is that each additional foot of hay will no longer provide the same stopping power as each previous foot as velocity and height increase (see graphs). Here is an addition from Matt M., who contributed that portion of the article:
The relationship between haystack height and stopping power is not a linear relationship. The greater the haystack height does not necessarily equal better stopping power. At some point, physics would argue that you hit an asymptote of effectiveness.

A haystack stopping a hero's fall is a commonly used movie device but one that, while very frequently used, has never been tested in real life. What sort of relationship exists between fall heights and haystack heights? How feasible is it for a haystack to stop someone falling from a great height? At what point is a haystack no longer effective at stopping someone? I've read that real haystacks sometimes use dead trees or logs to provide vertical structure to the pile, does that mean a realistic haystack would NEVER be a good cushion?