College Board-Aligned Original Notes
AP Physics C: Mechanics Unit 2 Topic 5: Newton's laws of motion: third law
Newton's Third Law describes paired forces between two interacting objects.
Unit 2: Force and Translational Dynamics. College Board exam weighting listed for this unit: 20%-25% of multiple-choice score.
What to Know
- Third-law force pairs are equal in magnitude and opposite in direction.
- The two forces act on different objects, so they never cancel in one object's free-body diagram.
- Third-law pairs are the same type of interaction, such as contact, gravitational, or tension-related interaction forces.
- Always connect this topic back to the larger unit: Force and Translational Dynamics.
Detailed Notes
Newton's laws of motion: third law belongs to Unit 2: Force and Translational Dynamics. Newton's Third Law describes paired forces between two interacting objects. In AP Physics C: Mechanics, the explanation should connect diagrams, calculus relationships, and the physical meaning of each quantity.
Key relationships: Third-law force pairs are equal in magnitude and opposite in direction. The two forces act on different objects, so they never cancel in one object's free-body diagram. Third-law pairs are the same type of interaction, such as contact, gravitational, or tension-related interaction forces.
Model to use: Name the interaction as force of object A on object B and force of object B on object A. Define the coordinate system, system boundary, and sign convention before writing equations.
AP-style move: When asked whether forces are equal, check whether the forces are a true interaction pair acting on different objects. Show the setup symbolically, then interpret the answer with units and direction when relevant.
Key Vocabulary
Third-law pair
Two equal and opposite forces from one interaction acting on different objects.
Mutual force
A force one object exerts as part of a paired interaction.
Different-object condition
The requirement that action-reaction forces act on two separate objects.
Contact interaction pair
A third-law pair caused by physical contact.
Gravitational interaction pair
A third-law pair caused by mutual gravitational attraction.
Quick Practice
How would you explain Newton's laws of motion: third law 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
- Center of mass
- Systems
- Newton's laws of motion: first and second law
- Circular motion
Official Alignment
Reference and Attribution
AP Physics C: Mechanics concept reference: AP Physics C Mechanics-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.