Types of Forces in Free Body Diagrams

Most mechanics problems use a surprisingly short list of forces. Learn the direction rule for each one, and drawing any FBD becomes mechanical.

1. Weight (W or mg)

Gravity acting on the object's mass.
Direction rule: always straight down (toward the Earth's center), drawn from the center of gravity. Magnitude W = mg, with g ≈ 9.81 m/s².

2. Normal Force (N)

The push from a surface that prevents objects from passing through it.
Direction rule: always perpendicular to the contact surface, pushing into the body. On a flat floor it points up; on a 30° incline it points 30° from vertical. A normal force can never pull.

3. Friction (f)

The resistance of a rough surface to sliding.
Direction rule: parallel to the surface, opposing relative sliding (kinetic, f = μkN) or the tendency to slide (static, f ≤ μsN).
Common trap: friction is not always "backwards" — for a car's driving wheels or a box on an accelerating truck bed, friction points forward because it prevents slipping.

4. Tension (T)

The pull transmitted by a rope, cable, string, or chain.
Direction rule: along the rope, always pulling away from the body. Ropes can only pull, never push. An ideal (massless, frictionless) pulley changes a rope's direction but not its tension — the same T acts on both sides.

5. Spring Force (Fs)

The restoring force of a stretched or compressed spring (Hooke's law: Fs = kx).
Direction rule: along the spring axis, always pointing back toward the spring's natural length — a stretched spring pulls, a compressed spring pushes.

6. Applied Force (F or P)

Any direct push or pull stated in the problem — a hand, a motor, a horizontal pull at 20° above horizontal.
Direction rule: exactly as given. Resolve into components along your axes when writing equations.

7. Distributed Load (w)

In statics, loads spread over a length — a beam's own weight, snow on a roof — measured in N/m or kN/m.
Direction rule: drawn as a row of arrows (or a shaded block) over the loaded length. For equilibrium calculations, replace it with a single resultant force equal to the area of the load diagram, acting at its centroid.

8. Air Resistance / Drag (FD)

Fluid resistance to motion.
Direction rule: opposite the velocity of the object relative to the fluid. Usually neglected unless the problem says otherwise.

Forces That Do NOT Belong on an FBD

• "Force of motion" / momentum — motion is not a force.
• Centrifugal force — in an inertial frame, circular motion needs a centripetal (inward) net force supplied by real forces like tension or friction; there is no outward force on the body.
• Internal forces — forces between parts of the same isolated body cancel and are omitted.
• ma — the net-force side of Newton's law, not an applied force.

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