for example i sometimes follow discussions about physics engines on several forums and mailinglists. read this post of the ODE author: http://q12.org/pipermail/ode/2004-May/012806.html

i know that such discussions aren't reliable sources but together with the conclusions i drew from my own experiments with several physics engines many of these things seem to make sense. if you do some stacking experiments you will see how fast you will bring a6's physics engine to its knees and that O(n^3) is plausible.