Free-Body Diagrams
Replace a complicated physical scene with one chosen system and the external force vectors acting on it. The simplification is the point.
How do we strip a messy scene down to only the forces that matter for one object?
Choose the system first. Then list the external interaction partners touching or influencing that system. Convert each interaction into one force vector. Everything else stays out of the diagram.
Motion arrows, future tendencies, and forces exerted by the system on other objects are not part of the free-body diagram.
A book resting on a level table.
1. System: choose the book.
2. Partners: Earth interacts gravitationally; the table interacts by contact.
3. Vectors: gravity points downward and the table's normal force points upward.
4. Omit: no “rest force,” no velocity arrow, and no book-on-table reaction force.
Which vectors belong on the sled?
A sled moves right across snow while a taut rope pulls it to the right.
A free-body diagram is not a picture of the scene. It is a bookkeeping model for external interactions acting on exactly one chosen system.
Velocity is not a force
Motion can be drawn separately when useful, but a velocity arrow does not belong in the force inventory.
Reaction pairs split across diagrams
The force your system exerts on another object belongs on that other object's free-body diagram.
Components are not extra forces
Breaking one angled force into x and y components is a mathematical rewrite of that force, not two additional interactions.