Use this as a quick reference for what counts as a force, the free-body diagram drawing conventions, and how to pick a smart set of axes.
🧭 Plot Summary
Every push and pull in this course turns out to need exactly two things: another object to push or pull, and a drawing convention clean enough to reason with. This lesson builds that convention from the ground up. A force is never something an object does to itself — it's always the result of an interaction with something else, whether that's a table pressing back on a book or a magnet reaching across empty space. The free-body diagramis the tool this course uses to track every one of those interactions at once: every force drawn as its own arrow, starting from one dot standing in for the whole system's center of mass. Get this representation right, and translating it into algebra — especially once you learn to choose your axes wisely — becomes almost mechanical.
What you'll do in this lesson
- Describe a force as an interaction between two specific objects or systems — never a thing an object does to itself.
- Classify contact forces as the macroscopic result of countless interatomic electric forces.
- Draw a free-body diagram showing only the forces the environment exerts on a chosen object or system.
- Represent every force as an individual vector originating from a dot at the system's center of mass.
- Follow the exam's drawing conventions: one arrow per force, no components, same-direction forces side by side.
- Choose a coordinate system with one axis parallel to the acceleration to simplify a diagram into algebra.
Why it matters
The free-body diagram you learn to draw here is the single tool nearly every later topic in this course leans on — Newton's three laws, friction, springs, circular motion, even torque diagrams in Unit 5 are all built on exactly this representation.
✅ Self-Check Before You Roll On
Check off each item as you get there. These aren't grades — they're your own signal.