To say it another way,
- within a portion of a beam's span Δx,
- if the difference in moments ΔM = Mb - Ma ≠ 0,
- then a horizontal shear force is developed on an internal surface of the beam to satisfy equilibrium.
What is another name for ΔM / Δx?
ΔM / Δx = V where V = transverse shear force.