Free-body diagrams
A picture of the forces acting on one chosen object.
What the diagram represents
A free-body diagram isolates a chosen object or system and shows the external forces acting on it. Each arrow gives a force’s direction; its length can represent magnitude when the scale is meaningful.
Name the source of every interaction. Gravity comes from the Earth in our examples. Normal forces, friction, and tension come from contact with something else.
A repeatable method
- State which object you are studying.
- Identify everything interacting with that object.
- Draw and label each force acting on it.
- Choose coordinate axes after you understand the forces.
Velocity and acceleration may be drawn separately, but they are not additional force arrows.
Check the boundary
If you include both a book and its table in one system, the contact forces between them become internal. If your system is the book alone, the table’s force is external.
Changing the boundary changes which interactions appear on the diagram. Be explicit about your choice.
A common mix-up
A pair of forces from Newton’s third law acts on different objects. Equal and opposite forces on a single object may balance, but they are not a third-law pair simply because their magnitudes match.
Use the connected problems below to test whether you can identify each force’s source.