Hello all, I am working on a science project based upon paper, and I have chosen the affect of friction between two interlaced books. The attachment below is something I have found on the internet, as part of my research. I don't understand why the Rav = 0.5 * (mass1 + mass2) *9.81. Why is the total mass of the books halved? Could you please explain this? Thanks in advance, A.T. EDIT: I am not sure if this is the correct section. If it isn't the correct section, please do let me know and I will delete this post and put it in the appropriate place.