Kinetic and Static Friction
Friction is a contact interaction parallel to a surface: static friction prevents relative slipping up to a maximum value, while kinetic friction acts when surfaces slide relative to each other.
What do I need to know for AP Physics 1 Topic 2.7?
Topic 2.7 Kinetic & Static Friction: Friction is a contact interaction parallel to a surface: static friction prevents relative slipping up to a maximum value, while kinetic friction acts when surfaces slide relative to each other.
- Determine friction direction from relative-motion tendency, distinguish actual static friction from its maximum, calculate kinetic friction, and account for changes in normal force.
- Practice target: 42 original MCQs and 4 original FRQs for this exact topic.
- Course alignment: current AP Physics 1 framework for the 2026–27 school year and May 2027 exam.
What is Kinetic and Static Friction?
Friction is a contact interaction parallel to a surface: static friction prevents relative slipping up to a maximum value, while kinetic friction acts when surfaces slide relative to each other.
Physics terms this topic teaches directly
Use these as working definitions for the lesson. Each definition is paired with a physical situation so the vocabulary is connected to observable motion or interactions rather than memorized in isolation.
Static friction
Static friction acts between surfaces that are not slipping relative to each other and adjusts to the value needed to prevent slipping, up to a maximum.
Maximum static friction
Maximum static friction is the largest possible static-friction magnitude in the ideal model, f_s,max=μ_sN; actual static friction can be smaller.
Kinetic friction
Kinetic friction acts when surfaces slide relative to each other; in the ideal model its magnitude is f_k=μ_kN and its direction opposes relative sliding at the contact.
Relative slipping
Relative slipping means the two contacting surfaces move relative to each other at the contact point; friction direction is determined by actual or impending relative slip, not simply by center-of-mass velocity.
Understand these ideas here before continuing
You should not have to leave this lesson just to decode its vocabulary. Each concept below is defined here, connected to the physics of this topic, and illustrated with a real situation. A related lesson link is provided only for deeper study.
Normal force
Meaning here: The contact force perpendicular to a surface; friction limits depend on its magnitude.
How this topic uses it: The ideal friction models contain N, so changes in the perpendicular support force can change the maximum static friction and the kinetic-friction magnitude.
Net force and acceleration
Meaning here: Friction contributes to the net external force and therefore to acceleration.
How this topic uses it: Friction is one contributor to the external force sum. Whether it speeds an object up or slows it down depends on the direction of the resulting acceleration relative to velocity.
Relative slipping
Meaning here: Friction direction is determined by actual or impending sliding at the contact, not simply by the object’s center-of-mass velocity.
How this topic uses it: Friction responds to motion or impending motion at the contact surfaces. Its direction cannot be chosen reliably by looking only at the center-of-mass velocity of the object.
Static vs kinetic
Meaning here: Static friction acts without sliding and adjusts up to a maximum; kinetic friction applies once surfaces slide.
How this topic uses it: The two models apply to different contact conditions. Static friction self-adjusts up to a maximum before slipping, whereas the ideal kinetic model applies after relative sliding begins.
How Kinetic and Static Friction works
Build the physical model first. The examples below connect the idea to situations you could actually observe before the page turns to equations.
Static friction adjusts to prevent slipping
Static friction is not automatically μ_sN. Its actual magnitude is whatever is required to prevent relative slipping, from zero up to the maximum μ_sN. If a 15 N horizontal push does not move a box whose maximum static friction is 40 N, the static friction is 15 N, not 40 N. On a parked car on a hill, static friction points in the direction needed to prevent the tires from slipping down the road surface.
Kinetic friction applies after sliding begins
When surfaces slide relative to each other, the simple AP model uses f_k = μ_kN. The direction opposes the relative sliding at the contact. The normal force N must be found from the actual vertical or perpendicular dynamics; it is not always mg. Pulling a suitcase upward at an angle can reduce N and therefore reduce kinetic friction, while pushing downward can increase both N and the friction magnitude.
The ideal dry-friction model has specific assumptions
The coefficients μ_s and μ_k depend on the pair of surface materials, and μ_s is typically larger than μ_k. In the ideal model, changing the apparent contact area alone does not change μN when the normal force and surface pair remain the same. Rolling without slipping can involve static friction because the tire and road are not sliding relative to each other at the instantaneous contact point; friction does not simply mean “a force opposite the object's velocity.”
Core ideas with real physical examples
Each category keeps the physics statement and the real situation together, so the page does not reduce the topic to abstract definitions.
| Idea/type | What it means | Real example 1 | Real example 2 |
|---|---|---|---|
| Static friction | Adjusts to the required magnitude needed to prevent slipping, up to a maximum. | A parked car on a hill can be held by static friction uphill. | A box pushed gently may remain still because static friction matches the push. |
| Maximum static friction | The threshold μ_sN; it is not the actual friction in every static situation. | If μ_sN=40 N and the push is 15 N, static friction is 15 N, not 40 N. | At the instant slipping begins, static friction reaches its limiting magnitude. |
| Kinetic friction | Approximately μ_kN in the simple sliding model. | A box sliding right on a stationary floor feels kinetic friction left. | A sled moving across snow loses speed because kinetic friction opposes its sliding. |
| Normal-force dependence | Changing perpendicular forces changes N and therefore friction magnitudes. | Pulling a suitcase upward at an angle reduces N and can reduce kinetic friction. | Pushing downward on a box increases N and raises the friction scale. |
| Contact area in the simple model | For the AP ideal dry-friction model, friction magnitude does not depend on the apparent area of contact when N and the surface pair are unchanged. | Turning the same rectangular block onto a smaller face does not by itself change μ_kN. | Wide versus narrow tire-contact drawings do not justify changing friction solely because the drawn contact area changed. |
| Coefficients depend on surface materials | μ_s and μ_k describe the interacting surface pair; typically μ_s is greater than μ_k for that pair. | Rubber on dry pavement can have a different coefficient from rubber on ice. | Once a heavy crate starts sliding, the kinetic-friction magnitude can be lower than the maximum static-friction threshold that had prevented motion. |
College Board focus for Topic 2.7
2.7.A–B · Describe static and kinetic friction between contacting surfaces.
Important boundary: Static friction adjusts up to μ_sN; it is not automatically equal to its maximum.
Equations, variables, SI units and when they apply
| Relationship | Variables / SI units | When to use it | Real example |
|---|---|---|---|
| 0 ≤ fs ≤ μsN | f_s,N: N; μ_s: unitless | Static-contact regime before slipping. | A 20 N push on a stationary box can be balanced by 20 N static friction if the maximum exceeds 20 N. |
| fs,max = μsN | f_s,max,N: N; μ_s: unitless | Threshold for impending slip. | With μ_s=.40 and N=100 N, maximum static friction is 40 N. |
| fk = μkN | f_k,N: N; μ_k: unitless | Sliding-contact regime. | With μ_k=.25 and N=100 N, kinetic friction is 25 N. |
Interactive Kinetic and Static Friction check
Static-vs-kinetic friction analyzer
Set normal force, coefficients and applied force. The tool identifies whether static friction can hold or sliding occurs.
Adjust a slider to explore the model.
Three real-situation worked examples
Gentle push
A 15 N horizontal push does not move a box whose f_s,max is 40 N.
Push exceeds threshold
The same box is pushed with 50 N.
Rolling tire
A tire rolls without slipping while a car accelerates.
Common AP mistakes—and the correction
| Common mistake | Correct physics | Real-life check |
|---|---|---|
| Setting f_s = μ_sN automatically. | That is only the maximum possible static-friction magnitude. | Gentle push: test the statement against this physical situation. |
| Saying friction always opposes object velocity. | It opposes relative slipping or the tendency to slip at the contact. | Push exceeds threshold: test the statement against this physical situation. |
| Assuming N = mg in every friction problem. | Angles, vertical acceleration, and other vertical forces can change N. | Rolling tire: test the statement against this physical situation. |
| Calling rolling-without-slipping friction kinetic. | No relative sliding at the contact means friction can be static. | Gentle push: test the statement against this physical situation. |
Master this topic beyond a formula list
This checkpoint combines the current AP Physics 1 content boundary with representation, experiment/data reasoning, real situations and cross-topic transfer.
2026–27 mastery checklist
- Distinguish static friction from kinetic friction based on relative slipping at the contact.
- Recognize that static friction self-adjusts up to a maximum magnitude, while kinetic friction uses its own coefficient model.
- Relate friction direction to the tendency for relative slipping rather than automatically opposing an object’s velocity.
- Combine friction with normal-force reasoning and Newton’s second law.
Must-not-miss relationship
0 ≤ f_s ≤ μ_sN and f_k = μ_kN in the standard model. Actual static friction is not automatically μ_sN.
Two anchor situations
A parked car on a modest incline can have static friction smaller than μ_sN because only enough friction to prevent slipping is needed.
A sliding box experiences kinetic friction opposite the relative sliding at the contact.
Diagram / graph / equation / claim
Draw a contact-point tendency arrow first, then choose the friction direction that opposes that tendency and verify with the full FBD.
How to test the physics
Measure the force needed to start and then maintain sliding of a block. Compare peak static-friction estimate with average kinetic-friction force and discuss scatter.
Coefficient models are idealizations; do not infer friction direction from motion alone, and do not set static friction to its maximum unless impending slip is established.
Use the static/kinetic distinction again when analyzing rolling without slipping versus rolling while slipping.
Connect friction to Topic 6.5 rollingGo beyond memorizing the equation
This topic now combines model selection, physical intuition, a worked transfer problem, graph/data analysis, answer verification, and the four current AP free-response reasoning modes.
Choose the model before calculating
Determine the normal force, then decide whether static friction can satisfy the needed value. Use f_s≤μ_sN until the threshold is exceeded; after sliding begins use f_k=μ_kN.
Why the physics works
Static friction is a responsive contact force up to a maximum, while kinetic friction is modeled with an approximately fixed magnitude proportional to normal force.
Solve this before opening the solution
A 5 kg block is on a horizontal surface with μ_s=0.40 and μ_k=0.30, using g=10 m/s². What friction acts for a 15 N horizontal push? What happens for a 25 N push?
Reveal complete reasoning
Solution: N=50 N. Maximum static friction is 20 N, so a 15 N push is balanced by 15 N static friction. A 25 N push exceeds the threshold; the block slides and kinetic friction is 15 N.
Physics check: Static friction should never be set equal to μ_sN unless the object is at the threshold of slipping.
What should you plot?
Measure kinetic friction for several normal forces and plot f_k versus N.
What does the graph mean?
The slope estimates μ_k. A separate threshold plot can estimate μ_s from maximum static friction.
How this exact topic can appear in each current FRQ mode
0 ≤ f_s ≤ μ_sN and f_k = μ_kN in the standard model. Actual static friction is not automatically μ_sN.
Draw a contact-point tendency arrow first, then choose the friction direction that opposes that tendency and verify with the full FBD.
Measure the force needed to start and then maintain sliding of a block. Compare peak static-friction estimate with average kinetic-friction force and discuss scatter.
Use the static/kinetic distinction again when analyzing rolling without slipping versus rolling while slipping.
State whether the object is sticking or sliding before choosing the friction equation, and justify the regime numerically.
Continue with the exact topic question banks
Use the focused banks for skill practice. The five full model papers are reserved for whole-exam simulation.
Topic 2.7 quick questions
What is the first thing I should identify in a Topic 2.7 problem?
Determine friction direction from relative-motion tendency, distinguish actual static friction from its maximum, calculate kinetic friction, and account for changes in normal force.
Should I memorize an equation before understanding the physical model?
No. Start by identifying the system, interactions, directions and representation. Then choose the relationship whose assumptions match the situation.
Where should I practice this exact topic?
Use the direct 42-MCQ and 4-FRQ Topic 2.7 banks on this page. The five-model-paper page is reserved for full-exam simulation.
Is this aligned to the May 2027 AP Physics 1 exam?
Yes. The course content remains based on the Fall 2024 framework for 2026–27; the Fall 2026 update changes the May 2027 exam timing and question count, not this Unit 2 topic sequence.
Where Topic 2.7 appears in AP-style released work
These are original topic summaries and alignment notes, not reproduced College Board question text. One released FRQ can legitimately connect to several topics.
Kinetic-friction investigation and linearization
Emphasizes experimental variables, friction modeling, graph construction and slope interpretation.
Experimental Design & AnalysisStatic-friction coefficient experiment
Direct experimental connection to static friction and graph/data analysis.
Experimental Design & AnalysisRecent public FRQs are the safest format reference. Older released questions can still contain excellent physics, but the course and exam changed; use the atlas alignment notes before treating a legacy question as a current-format simulation.