Question: I am trying to pick a physics engine for a simple software application. It would be to simulate a rather small number of objects so performance isn't a huge concern. I am mostly concerned with the accuracy of the motion involved. I would also like the engine to be cross-platform between windows/linux/mac and usable with c++ code. I was looking at Bullet, Newton Game Dynamics, and ODE because they are open source. However, if Havok/PhysX are significantly more accurate I would consider those too.

All I seem to find are opinions on the engines, are there any thorough comparisons between the options? Or does anyone have experience trying the various engines out. Since what I'm trying to do is relatively simple there probably isn't a huge difference between them, but I'd like to hear what people have to say about the options? Thanks!

Answer: 
Sorry, but you will never find a real comparison with respect to accuracy. I am searching for three months now for my master thesis and have not found it. So I started to do the comparison on my own but it's still a long way to go. I'm testing with 3d engines and even 2d engines and for now Chipmunk is the one with the highest accuracy so far. So if you have no need for 3d I would reccomend it. However if you have an urgent need for 3d and your problem is as simple as you described it (don't want to expand it in the future?) Bullet and ODE will do it. I would prefer Bullet because it is much more up-to-date and is still actively maintained. At least there is Newton, with which I am fighting right now. Therefore I can't give you pros and cons except that it is a bit more work to get familiar with the (crucial) bad documentation.
Hope that helps. Best regards.
A physics abstraction layer supports a large number of physics engines via a unified API, making it easy to compare engines for your situation.
[PAL](http://pal.sourceforge.net/) provides a unique interface for these physics engines:

1. Box2D (experimental)
2. Bullet
3. Dynamechs(deprecated)
4. Havok (experimental)
5. IBDS (experimental)
6. JigLib
7. Meqon(deprecated)
8. Newton
9. ODE
10. OpenTissue (experimental)
11. PhysX (a.k.a Novodex, Ageia PhysX, nVidia PhysX)
12. Simple Physics Engine (experimental)
13. Tokamak
14. TrueAxis

According to the [December 2007 paper](http://www.adrianboeing.com/pal/papers/p281-boeing.pdf) linked in [this answer](https://stackoverflow.com/a/13513801/819417):

> 
> Of the open source engines the Bullet engine provided the best results
>  overall, outperforming even some of the commercial engines. Tokamak
>  was the most computationally efficient, making it a good choice for
>  game development, however TrueAxis and Newton performed well at low
>  update rates. For simulation systems the most important property of
>  the simulation should be determined in order to select the best
>  engine.
> 
> 
> 

Here is a September 2007 demo by the same author:

<https://www.youtube.com/watch?v=IhOKGBd-7iw>