A: The deflection equation for a distributed load uses weight per unit length (lbf/in), not total weight (lbf) — that's why the units come out correctly when you carry them through. For a concentrated point load the formula uses total weight directly, so make sure you're applying the right form of the equation for your loading condition. The photograph you annotated shows the variable correctly; the units in the denominator reconcile once you recognize it's a distributed load formulation.