LÖVE

If an object is rubbing against an object, the friction between the two objects won't update until the collision ends. Not sure if this is a box2d issue or a love2d issue because I was looking at the method used here http://www.badlogicgames.com/wordpress/?p=2017 for player physics and it seems to work there, but it's not possible to change the friction of the player against the ground in love2d because the friction doesn't update.