Free-body diagram

A free-body diagram depicts an isolated object and the external forces and moments acting on it. It supports analysis of the object's motion or equilibrium.

Newton's laws of motion: Three laws relating an object's motion to the forces acting on it. They turn the forces drawn on the isolated object into equations of motion.
System isolation: The modeling step that separates a chosen body or system from its surroundings. A free-body diagram begins by choosing exactly what the diagram treats as the system.
Static equilibrium: A state in which a body has zero linear and angular acceleration. Its diagrams lead to force and moment balance equations.
Kinematic diagram: A simplified drawing showing the geometry and motion of a mechanical system. It represents motion and constraints, whereas a free-body diagram represents external loading.
Force: An interaction that can change an object's motion, measured as a vector quantity. Each force arrow represents an external interaction acting on the isolated object.
Normal force: A contact force exerted perpendicular to a surface at the point of contact. It represents the surface's push on an isolated object resting against it.
Projectile motion: The motion of an object moving through the air under gravity, with air resistance neglected. A diagram isolates the projectile and shows gravity as its sole force in the ideal model.
Shear and moment diagram: A plot of internal shear force and bending moment along a structural member. It is derived from loading analysis and shows internal effects, not just external actions.
Torque: The rotational effect of a force about a point or axis. A moment on the diagram represents a tendency to rotate the isolated body.
Static friction: A contact force that resists slipping between surfaces that are not sliding relative to each other. Its direction opposes impending relative motion, not necessarily the object's overall motion.

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