This is a GRE question, but it's so basic I thought I'd just put it here. Here it is with the solution: http://grephysics.net/ans/9277/6 The answer is "D" by the way. So my problem is with the way the weight of the block is distributed. I get that each triangular block only "feels" half of the square block, so M/2 of mass. But then they assume that's the horizontal force via some weird trig that seems pointless (I can't figure out where they get a square-root 2 from). My understanding was that you get half of the block's weight pushing down on the triangular block, and half pushing it horizontally. So you'd have g*M/4 pushing straight down and (m+M/2)*g*u pushing horizontally, for each block. Why am I wrong?