1. The problem statement, all variables and given/known data The grains of the wooden member forms an angle of 15° with the vertical. For the state of the stress shown, determine (a) the in-plane shearing stress parallel to the grain, (b) the normal stress perpendicular to the plane. I have attached an image of the question. 2. Relevant equations τx'y' = (-σx-σy)/2*sin(2θ) + τxycos(2θ) 3. The attempt at a solution I'm having some trouble understanding what is going on here. I can see that the is a 600 psi shearing stress applied at the sides and that there are no normal stresses, hence the first portion of the equation I gave above is zero, leaving me with: τx'y' = τxycos(2θ) where τxy is 600 psi. Hence I get: τx'y' = 600cos(2*15) = 520 psi which i know is correct Can someone describe what is physically happening? For part b I know the answer is - 300 psi which I can get from: τx'y' = τxysin(2θ) = 300 psi but what makes it negative? and why does the cos change to a sin?