Free body diagram

A free body diagram (FBD) isolates one object from its surroundings and shows every external force acting on it as a vector arrow. The object is drawn as a simple dot or box, and each arrow points in the direction the force acts, with its length suggesting relative magnitude. Forces the object exerts on other things are never included — only forces acting on the object.

Consider a box resting on a ramp. Its free body diagram shows the weight force mg pointing straight down, the normal force perpendicular to the ramp's surface, and friction acting along the surface, opposing sliding. If a rope pulls the box, a tension force is added along the rope's direction. Common forces to check for include gravity, normal force, tension, friction, applied pushes or pulls, and spring forces.

The diagram's purpose is to set up Newton's second law, F = ma. After drawing the FBD, you choose coordinate axes (often tilted to match a ramp), resolve forces into components, and write ΣF = ma along each axis. If the object is in equilibrium — at rest or moving at constant velocity — the net force is zero and the equations become ΣF = 0. Most errors in force problems trace back to a flawed diagram: a missing force, an invented force like a leftover "motion force," or a misdirected normal force.

The AP Physics 1 exam requires free body diagrams explicitly — both multiple-choice and free-response questions ask students to draw or identify correct FBDs before applying Newton's laws, and rotational problems extend the same skill to identifying where forces act for torque calculations.