There is not really a homework problem. I just want to understand the concept here. See the image attached. How is it that in the free body diagram of the image shown the angle theta is from FN to the perpendicular Force applied by box to the horizontal? It is hard to describe in words. I just want to know how am I placing my angles and X,Y in a free body diagram of this setup. I always thought the X axis was Cos and Y was Sin but when my professor solved the problem, FN, which is vertical, was determined by FN cos(theta) I dont understand.