Why
Purpose of Free-Body Diagrams
FBDs separate the object from its environment so you can model interactions clearly.
- What an FBD shows
- What it does not show
- When to draw one
Dynamics · Physics
Free-Body Diagrams
The universal translation: physical situation → forces → equations → motion.
This topic includes
Subtopics to master
Learn to isolate an object, list forces correctly, choose axes, and write ΣF equations cleanly.
Step 1
Isolating the Object of Interest
Pick the “system” you’re analyzing and ignore internal details.
- Choose the system boundary
- One object vs multiple objects
- System vs surroundings
Step 2
Identifying All Forces Acting
List all external forces: gravity, normal, tension, friction, drag, applied forces, etc.
- Contact vs non-contact
- Forces come from interactions
- Common “missing force” errors
Step 3
Choosing Coordinate Axes
Choose axes that simplify component calculations (often along motion or along a surface).
- Smart axis choices
- Incline-aligned axes
- Sign conventions
Translate
From FBD to Equations
Convert the diagram into ΣFx = m ax and ΣFy = m ay (or equilibrium).
- Component breakdown
- Equilibrium case ΣF=0
- Check units and signs
Practice
Practice & Exercises
Build fluency: draw FBDs fast and write correct ΣF equations.
- FBD identification drills
- “List all forces” mini sets
- Axis-choice practice
- Convert to equations
- Mixed multi-step problems