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: 
Although it is a bit dated, there is a comprehensive comparison of (in alphabetical order) Bullet, JigLib, Newton, ODE, PhysX, and others available here:

* <http://www.adrianboeing.com/pal/papers/p281-boeing.pdf>

The comparison considers integrators, friction models, constraint solvers, collision detection, stacking, and computational performance.
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>