   Friction between two surfaces
  When I have two surfaces with different friction koefficients, how can I dynamicly calculate the final frictional force working on the surfaces?   
  There won't be two different friction coefficients. A single surface has no friction coefficient. A friction coefficient is defined between pairs of surfaces, and this is essentially determined experimentally. You can probably find tables of friction coefficients between different types of surface pairs, using an internet search.Now, I said there aren't two different coefficients. Friction is complex. The friction coefficient (and therefore force) is different depending on whether the surfaces are sliding past one another (kinetic case) or not sliding/stuck together (static case). So, simple simulations typically have separate kinetic and static friction coefficients for a given pair of surface types. There are  more sophisticated approaches to deal with continuous changes in the relative  motion of the two surfaces, and you can google "tribology" to look into the science of this.Hope that helps!