Sorry about that. I really ought to upgrade to 9.

Apparently Mathematica added some new features in version 9 that are intended to better handle differential equations with discontinuities (see http://reference.wolfram.com/mathematica/tutorial/NDSolveWhe...). Apparently this is getting tripped up at the boundary between the two behaviors.

One simple thing that might work is to change the force law to either

  force[disp1_, disp2_, dir_] := -k Abs[(disp1 - disp2).dir ]^(3/2) dir* If[(disp1 - disp2).dir > 0, 1, 0]

or

  force[disp1_, disp2_, dir_] := -k Abs[(disp1 - disp2).dir ]^(3/2) dir* Piecewise[{{1, (disp1 - disp2).dir > 0}}, 0]

If these don't fix it, then you would need to play around with the options for NDSolve.

Thanks for the help. Piecewise has the same problem, whereas the If is taking forever. How long should I wait before I kill it?

There's a Mathematica 10 now, already? Interesting.

I'm still a happy 8 user, myself. Is there much reason to upgrade?

It'd probably be good for them to pay more attention to backwards compatibility...

10 has Association expressions [1], which are a huge language addition (and long overdue). Seems there is some synchrony with Erlang on that front!

The features I'm most excited about in 10 (disclosure: my team is working on some of these):

[2] String, XML, file templating

[3] Hands-free machine learning

[4] Entity and EntityValue: semantic "hooks" to represent real world entities

[5] Dataset, a general data modelling and query framework.

Also, DateObject, TimeObject, interaction with external processes, multiple undo, a large number of updated data sources, the ExternalService "meta-api", device connections, time series stuff, GeoGraphics, and a long list of no less worthy features.

[1] http://reference.wolfram.com/language/guide/Associations.htm...

[2] http://reference.wolfram.com/language/guide/WorkingWithTempl...

[3] http://reference.wolfram.com/language/guide/MachineLearning....

[4] http://reference.wolfram.com/language/ref/Entity.html

[5] http://reference.wolfram.com/language/ref/Dataset.html

Nice to see all the WolframAlpha features being brought over. I'd completely missed Entity and Dataset.

Any chance you'll be including the templating features in the RPi version?

Yup, although there is a bit of design change from the templating in the RPi version.

I think he is confused. Wolfram's site only has Mathematica 9 listed

> I think he is confused.

You don't mean that the GP (taliesinb) is confused, do you?. He works for Wolfram Research, developing Mathematica.

That is what I meant, but I guess I look like the fool now. I still don't see that Mathematica 10 has been released, but I guess it is close.

It hasn't been released, but you can effectively get it on a Raspberry Pi: http://wolfram.com/raspi

I'm going to go out on a limb and guess that v. 10 has not yet been released but is in common use internally at Wolfram.

That is odd. Mathematica is usually good about backwards compatibility. That sounds like a change in the under lying numerical solver.

Well, if it was a lying numerical solver...

Seriously: I think they will make breaking changes if the existing code turns out to be mathematically incorrect. That may or may not have been the case here.