College Board-Aligned Original Notes
AP Physics 1: Algebra-Based Unit 2 Topic 2: Forces and free-body diagrams
A free-body diagram isolates one object and represents every external force acting on it.
Unit 2: Force and Translational Dynamics. College Board exam weighting listed for this unit: 18%-23% of multiple-choice exam score.
What to Know
- Forces are interactions, so every force should have a source object and a receiving object.
- Only forces acting on the chosen object appear in that object's free-body diagram.
- Net force is the vector sum of all external forces and determines acceleration.
- Always connect this topic back to the larger unit: Force and Translational Dynamics.
Detailed Notes
Forces and free-body diagrams belongs to Unit 2: Force and Translational Dynamics. A free-body diagram isolates one object and represents every external force acting on it. Start by naming the system, choosing axes or a sign convention when needed, and identifying which physical quantity is changing.
Key relationships: Forces are interactions, so every force should have a source object and a receiving object. Only forces acting on the chosen object appear in that object's free-body diagram. Net force is the vector sum of all external forces and determines acceleration.
Model to use: Represent the object as a dot or simple shape, then draw labeled force arrows from the object outward. A strong model should include labels, directions, units, and the reason each relationship applies.
AP-style move: Do not draw velocity or acceleration as forces; use them separately to reason about motion. For free-response work, write the principle first, show the setup, and then interpret the result in the physical context.
Key Vocabulary
Free-body diagram
A diagram showing all external forces acting on one selected object.
Applied force
A push or pull exerted by an object or agent.
Normal force
A contact force perpendicular to a surface.
Tension force
A pulling force transmitted through a string, rope, or cable.
Net force
The vector sum of all external forces on an object.
Quick Practice
How would you explain Forces and free-body diagrams in one or two AP-style sentences?
Name the concept, apply it to a specific example or source, and explain the reasoning that connects the evidence to your answer.
Related Topics in This Unit
- Systems and center of mass
- Newton’s Third Law
- Newton’s First Law
- Newton’s Second Law
Official Alignment
Reference and Attribution
AP Physics 1 concept reference: AP Physics 1: Concise Summary Notes by IITian Academy. EduCompanion used this source as a topic checklist and conceptual reference, then rewrote the study notes in original wording.