Concept
Internal forces — where it appears
The axial force, shear, moment and torque a cut reveals, which exist because the free body on either side of it has to be in equilibrium.
Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.
The free body is a choice, and choosing it well is the whole skill
Cutting a structure open is not a step in the method. It is the method — and where the cut is made decides whether the answer takes one line or twenty.
The internal force with no diagram
A cut through a member reveals four things, and this collection has drawn diagrams for three of them. The fourth is a torque, it obeys exactly the same rules, and whether it exists at all can depend on a decision the designer is free to make.
Named alongside it
The objects these essays reach for when they reach for this one.
BoundaryCompatibility torsionEquilibriumFree body diagramMethod of sectionsNewton's third lawReactionsShear centreShear flowStiffnessTorsionTorsion constant