Let's assume that the force of friction is only present between the two blocks and that the pulley is massless. On top of that, force F pulling the two blocks at a constant velocity; as a result, the acceleration = 0m/s2. May someone tell me how many forces of friction are going on between the "2kg" and "7kg" blocks? I guessed two since the top block is moving left and the bottom box is moving right. Am I right? And is it true that the force of gravity that applies on the "7kg" block is actually (2kg+7kg)*g = 9kg*g? Also may you please check if my sum of all forces are corrrect?: ΣFtop blockx: -T1 + μ*2kg*g = [2kg*0] -> T1 = μ*2kg*g ΣFtop blocky: Ntop = 2kg*g ΣFbottom blockx: T2 - μ*9kg*g - T1 + F = [9kg*0] -> T2 = μ*9kg*g + 1 - F ΣFbottom blocky: Ntop = 9kg*g note: sorry but I don't know if I should still multiply kg * gravity or just leave it as kg.