Okay, here is the problem I am working on... A block of a certain weight(w2) is sliding down an inclined plane at angle theta. There is a plank on top of the block with a certain weight (w2) that is attacked to a cord at the top of the plane so that it does not move. I need to find the coefficient of kinetic friction in terms of the masses of each block and the angle theta. The coeffecient is the same between both surface contacts. I am using Newton's 2nd law and I'm stuck with the friction. In my free body diagram I am not sure which direction the force of friction is going between the block and the plank nor how it affects both.