College Board-Aligned Original Notes
AP Physics 1: Algebra-Based Unit 2 Topic 7: Kinetic and static friction
Friction is a contact force parallel to a surface that resists slipping or the tendency to slip.
Unit 2: Force and Translational Dynamics. College Board exam weighting listed for this unit: 18%-23% of multiple-choice exam score.
What to Know
- Static friction adjusts up to a maximum value before slipping begins.
- Kinetic friction acts when surfaces slide relative to each other.
- Friction depends on the normal force and the coefficient for the surface pair in the model.
- Always connect this topic back to the larger unit: Force and Translational Dynamics.
Detailed Notes
Kinetic and static friction belongs to Unit 2: Force and Translational Dynamics. Friction is a contact force parallel to a surface that resists slipping or the tendency to slip. Start by naming the system, choosing axes or a sign convention when needed, and identifying which physical quantity is changing.
Key relationships: Static friction adjusts up to a maximum value before slipping begins. Kinetic friction acts when surfaces slide relative to each other. Friction depends on the normal force and the coefficient for the surface pair in the model.
Model to use: Decide whether the surfaces are sliding, then draw friction opposite relative motion or impending motion. A strong model should include labels, directions, units, and the reason each relationship applies.
AP-style move: Static friction is not automatically equal to coefficient times normal force; that expression gives the maximum possible static friction. For free-response work, write the principle first, show the setup, and then interpret the result in the physical context.
Key Vocabulary
Static friction
Friction between surfaces that are not sliding relative to each other.
Kinetic friction
Friction between surfaces sliding relative to each other.
Coefficient of friction
A unitless value describing friction strength for a pair of surfaces.
Impending motion
The direction an object would begin to slide without enough static friction.
Frictional dissipation
Conversion of mechanical energy into thermal energy through friction.
Quick Practice
How would you explain Kinetic and static friction 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
- Forces and free-body diagrams
- Newton’s Third Law
- Newton’s First 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.