Q is a special section property that is used with V and I in the shear flow equation to compute the unbalanced longitudinal force per unit length of beam for a specific portion of the cross section.

To compute Q, we focus on that part of the beam shape that must be connected to the rest of the cross section. For the beam in this example problem, board (3) must be connected to boards (1) and (2) by nails. Therefore, we compute Q for board (3).