How to Draw a Free Body Diagram in 6 Steps

Drawing a free body diagram (FBD) is a skill, and like any skill it follows a repeatable process. Use these six steps on every problem — from a block on a table to a loaded bridge truss — and you will never miss a force.

Step 1 — Isolate the Body

Decide exactly what object you are analyzing, and mentally cut it away from everything touching it. Draw it as a simple shape: a box for a block, a line for a beam, a dot for a particle. Do not draw the ramp, rope, or wall — they will be replaced by forces.

Tip: if two objects move together (e.g., stacked blocks), you may treat them as one body — or isolate each separately when you need internal contact forces.

Step 2 — Add the Weight

Every object with mass has weight. Draw one arrow, labeled W or mg, pointing straight down from the center of gravity. This is usually the only non-contact force in introductory problems.

Step 3 — Replace Every Contact with a Force

Walk around the boundary of your isolated body. Everywhere something was touching it, add the force that contact provides:

• Surface touching the body → normal force N, perpendicular to the surface (plus friction f along the surface, if it's rough).
• Rope, cable, or string → tension T, pulling away from the body along the rope.
• Spring → spring force Fs, along the spring axis (pushing or pulling depending on compression/stretch).
• Support (statics) → reaction forces: a roller gives one perpendicular reaction; a pin gives two components (Rx, Ry); a fixed support gives two components plus a moment (M).

Step 4 — Add Applied Loads

Add any pushes, pulls, distributed loads, or applied moments given in the problem, at the points where they act, with their known magnitudes and angles.

Step 5 — Choose Coordinate Axes

Draw a small x–y axes symbol next to the diagram. For inclined surfaces, tilt the axes so x runs along the incline — this turns weight into components (mg sin θ and mg cos θ) and keeps everything else on-axis, which greatly simplifies the algebra.

Step 6 — Check the Diagram

Before writing equations, verify:

• Every arrow starts on the body and is labeled.
• Count the touches: number of contacts = number of contact forces (plus weight).
• No "motion force" or "centrifugal force" arrows — motion is a result of forces, not a force.
• For equilibrium problems: could the arrows plausibly balance? If every arrow points the same way, something is missing.

Worked Example: Block on a 30° Incline

1. Isolate: draw a tilted box.
2. Weight mg straight down.
3. Contacts: the incline surface → normal N perpendicular to the slope, friction f along the slope (up-slope if the block tends to slide down).
4. No other applied loads.
5. Axes tilted along the incline.
6. Check: 3 arrows, all labeled — done. The equations follow: N = mg cos θ, f = mg sin θ (if static).

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