   implementing rolling friction
  Whilst static friction is incredibly easy to implement, simply using a different coeffecient when the relative velocity is below a threshold and letting my calculations go on just the same, i'm not quite sure how best to go about rolling friction as in the case of say a circle rolling, the relative velocity is infact 0 and so normal friction does nothing to slow the objects motion but adding a seperate frictional impulse considering say the bodies relative motion at their centres as opposed to the contact point doesn't sound like a good idea.   
  Whilst not perfect, using polygonal based circles, instead of 'actual' circles, rolling friction would come about naturally, and since it really isn't a bottleneck (collision that is) it doesn't really effect framerate at all.(if anyone is interested (though this doesn't show rolling friction at all really)) here's a nice little example using my engine:http://www.newgrounds.com/dump/item/516a948b8cd4d18edcb028f7db425116click to generate objects.(Due to the way sleeping is performed etc etc, generating objects inside a large sleeping stack may cause objects to not be fully seperated; but i don't really intend on changing that since in any application it's not something which will be going on)   ;  http://en.wikipedia.org/wiki/Rolling_resistance   ;  That really doesn't help, I know what rolling friction is, the question is how to implement it. for example normal friction is implemented as a constraint that serves to make the relative velocity at the contact 0, limited by the normal force.   