Box2D on iOS

I need a Cocos2D Scene where I have a Box2D world with zero gravity, and I need sprites to attract each other so they can cluster together. Imagine marble balls (so it's rigid body) that have a mass and have their own gravity field. The rest is up to me, I only need this Scene working over opengl rendering and that's all. For paranoia sake, all marble balls must have a 100 pixels radius (I'm not sure how many meters do this make in Box2D ).