{"id":1808,"date":"2026-08-29T00:20:22","date_gmt":"2026-08-29T00:20:22","guid":{"rendered":"https:\/\/physicsfundamentalsinfo.com\/blog\/?p=1808"},"modified":"2026-08-29T21:39:10","modified_gmt":"2026-08-29T21:39:10","slug":"free-body-diagram","status":"publish","type":"post","link":"https:\/\/physicsfundamentalsinfo.com\/blog\/mechanics\/free-body-diagram\/","title":{"rendered":"Free Body Diagram: How to Draw One Step by Step"},"content":{"rendered":"\n
Definition<\/div>

\nA free body diagram is a simplified sketch that shows one chosen object as a dot or box, with every external force acting on it drawn as a labelled arrow. It strips away the surroundings, so the vector sum of those arrows gives the net force \u2014 which equals mass times acceleration.\n<\/p><\/div>\n\n

You already do this without noticing. Push a stalled car and you feel exactly three things that matter: your push, the ground dragging back, and the car’s stubborn weight pressing down. Your brain quietly discards the paint, the passengers, the shopping in the boot.<\/p>

A free body diagram is that instinct, made rigorous. Get it right and a page of confusing physics collapses into two short equations. Get it wrong \u2014 one stray arrow, one missing contact \u2014 and every number after it is wrong too.<\/p>

What Is a Free Body Diagram?<\/h2>

A free body diagram is a drawing of a single object showing only the external forces acting on it, with each force drawn as an arrow starting at the object. Nothing else appears: no surfaces, no ropes, no neighbouring blocks.<\/p>

The word free<\/em> is doing real work here. You are freeing the body from its surroundings, then replacing every removed contact with the force it was exerting. The floor vanishes and becomes an upward arrow. The rope vanishes and becomes a pull along its old direction.<\/p>

Why bother? Because forces obey vector arithmetic, and vector arithmetic needs a clean list. As soon as the picture contains a wall, a pulley and three blocks, you cannot tell which forces belong to which object.<\/p>

One body, one diagram<\/h3>

This is the rule beginners break most often. If a problem has two blocks, it has two free body diagrams \u2014 never one crowded picture with arrows going everywhere.<\/p>

The payoff is that each diagram gives you its own equation. Two diagrams, two equations, two unknowns: the system solves.<\/p>

The Free Body Diagram Formula<\/h2>

Every free body diagram exists to feed one equation \u2014 Newton’s second law, applied separately to each axis.<\/p>\n\n

\u03a3F = ma<\/div>\n\n

In practice you never use it in that vector form. You split it the moment you have chosen axes:<\/p>\n\n

\u03a3Fx = max and \u03a3Fy = may<\/div>\n\n