Question for a new project

Im working on a new project with gravity reversing as a key component. I'm using NxOgre at the moment, but im not sure its possible to do what I want with it, so I'm just going to ask this on the forums for all the physics wrappers. I need to be able to reverse gravity on one object at a time, but they still need to be able to interact with each other. So my question is basically can I do this with Bullet?

AFAIK gravity is applied just like any force in Bullet (probably in PhysX too), so you can do it yourself without losing any efficiency. Just set gravity to 0 and add a force to each active body every timestep (or every frame, but then you have to take the frame time into account since it is not constant - gravity*elapsedtime).

Alternatively I suppose you can add a force of 2*gravity*elapsedtime in the inverse direction of gravity, but this seems a bit hacky... might cause unstable simulation especially if your timestep/frametime is not the same as Bullets.