Statics

Free-Body Diagram

A sketch of one isolated body showing every external force and moment acting on it.

A free-body diagram isolates a single body from its surroundings and replaces everything that touched it with the force or moment that contact exerted. Supports become reaction forces, cables become tensions, and contact surfaces become normal and friction forces.

It is the first step in almost every statics problem, and drawing it carelessly is the most common reason an otherwise correct solution comes out wrong. Every force that crosses the boundary you drew must appear, and nothing internal to the body should.

How much of the FE Civil exam is Statics?