A Free Body Diagram (FBD) is a simplified sketch of a single object, isolated from all surroundings, showing every external force acting on it as a vector arrow. FBDs are the foundation of equilibrium analysis and Newton's second law problems.
Weight (W)
Gravitational pull mg, always pointing straight downward through the centre of mass.
Normal (N)
Contact force perpendicular to the support surface, preventing the object from passing through it.
Friction (f)
Contact force parallel to the surface, opposing motion or tendency of motion.
Tension (T)
Pull transmitted through a rope or cable, always directed away from the object along the cable.
Applied (F)
A direct push or pull exerted on the object by a person or another body.
Reaction (R)
Support force from a pin, roller, or wall that maintains structural equilibrium.
Reaction Moment (M)
A rotational (turning) reaction supplied by a fixed/built-in support, drawn as a curved arrow instead of a straight one.
Most supports (pins, rollers, smooth contacts) can only push or pull — they supply force reactions. A fixed support (also called a built-in, clamped, or cantilever support — for example a beam welded or bolted into a wall) is different: it also resists rotation. That rotational resistance is the reaction moment, drawn on an FBD as a curved arrow looping clockwise or counterclockwise around the support point, labelled M.
A simple rule of thumb: count the directions a support restrains.
- Roller — restrains motion in one direction only → one force reaction, no moment.
- Pin — restrains motion in two directions but allows free rotation → two force reactions, no moment.
- Fixed support — restrains motion in two directions and rotation → two force reactions plus a reaction moment.
- Identify the object of interest — this is the body you are "freeing".
- Draw the object as a simple shape (box, circle, or dot) in isolation.
- List every external agent that touches the object or acts at a distance.
- Draw each force as an arrow at the correct point of application, in the correct direction.
- Label every arrow with its symbol (W, N, f, T, F, R) and magnitude if known.
- Check completeness: weight is always present; support forces must match constraints.