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A free-body diagram isolates an object or system and shows all external forces and moments acting on it, removing all supports and connected bodies. Proper identification and representation of all forces—including reactions from supports—is essential for equilibrium analysis and is the critical first step in solving mechanics problems.
Practice on simple objects, then progress to complex systems. Always draw and label every force clearly: applied loads, weights, normal forces, friction, and reaction forces. Check that the diagram is truly isolated and includes nothing internally connected.
Every statics and dynamics problem reduces, at some point, to answering the question: what forces act on this object? A free-body diagram (FBD) is the systematic procedure for answering that question. The core act is *isolation* — you mentally cut away everything connected to the object of interest and replace each connection with the force or moment it was exerting. What remains is a sketch of the object alone, surrounded only by labeled force vectors.
Consider a book sitting on a table. The book's FBD has two forces: its weight W pulling downward (a body force from gravity) and a normal force N pushing upward from the table surface. The table itself does not appear in the diagram — only the force the table exerts. If you also press your finger on the book, a third force appears. The FBD is complete when every external agent that physically touches the object (or acts at a distance, like gravity) is represented, and nothing else. Internal forces — say, the binding holding the book's pages together — never appear, because they cancel in pairs within the body.
Supports and connections translate into specific force and moment types. A pinned support can push or pull in any direction, so it contributes two unknown force components (Fx and Fy in 2D). A roller only pushes perpendicular to the surface it rolls on — one unknown. A fixed wall support prevents both translation and rotation, so it adds two force components *and* one reaction moment. Learning these translation rules is as important as the drawing itself, because they determine which unknowns to solve for in your equilibrium equations.
The power of the FBD comes from what it enables: you can apply Newton's second law (or, for statics, the equilibrium conditions ΣF = 0 and ΣM = 0) directly and cleanly to the isolated object. A missing force means a wrong equation; an incorrectly replaced connection means a wrong unknown count. Practitioners sometimes draw the FBD before writing any equations at all — not as a formality, but because an accurate diagram makes the algebra almost mechanical. In problems with multiple bodies, you draw a separate FBD for each, and forces at shared surfaces appear as equal-and-opposite action-reaction pairs across diagrams.