Free Body Diagrams

A free body diagram (FBD) shows all the external forces acting on a single object. You represent the object as a point at its center of mass and draw every force that acts on it.

To construct an FBD, identify all forces on the object. Contact forces occur where something touches the object—a surface pushing, a rope pulling, or friction. Long-range forces act at a distance, like gravitational attraction. Each force is drawn as an arrow starting from the center-of-mass point. The arrow points in the direction the force acts. The length represents the magnitude.

Label each force clearly. Use notation that identifies what exerts the force and its type. For example, $\vec{F}_{\text{rope}}$ for the pull of a rope, $\vec{F}_{\text{surface}}$ for the push of a surface, or $\vec{F}_{\text{Earth}}$ for gravitational pull. At this stage, just draw and label what you observe—don't assume relationships between forces.

Choose a coordinate system (usually horizontal and vertical axes) to describe directions. Right now you're focused on representing the forces as vectors, not applying any laws or solving equations.

Below is an example FBD for an object resting on a horizontal surface with a rope pulling at an angle: