1. The problem statement, all variables and given/known data I'm looking at some problems and solutions to them, and when they calculate shear stress instead of having the area of the thing be pi*r^2, there is usually a (pi/4)*r^2. 2. Relevant equations Shear Stress = V/A 3. The attempt at a solution I don't understand where they get the 1/4 in the denominator.