What is the difference between the formulas $\tau = VQ/It$ and $\tau = V/A$ for finding shear stress due to transverse loading? I understand that the use of first moment of area, is the only reason for the more complex formula to account for shape?
For a square cross section the maximum shear stress used for failure analysis is simply 4V/3A, which relates to tau = V/A except it has a multiplication factor of 1.33. What is the point of this safety factor, since the rectangle is such a simple shape, and which formula should I use? The factory of safety ends up being very different due to the extra 1/3.