Kinetic friction is the contact force that opposes the sliding motion of an object already moving across a surface; its magnitude equals the coefficient of kinetic friction (μk) times the normal force, and it always points opposite the direction of sliding.
Kinetic friction is the resistive contact force between two surfaces that are sliding past each other. Once an object is actually moving, kinetic friction takes over from static friction, and its magnitude is simple to compute. It equals μk (the coefficient of kinetic friction, a number that depends on the two materials) multiplied by the normal force pressing the surfaces together. The direction is always opposite the object's sliding motion relative to the surface.
Here's the part that makes problems easier than you'd expect. Unlike static friction, kinetic friction is constant while the object slides. It doesn't care how fast the object moves or how hard you push. A block sliding at 1 m/s and the same block sliding at 5 m/s feel the same kinetic friction force. That constant force means constant acceleration (or deceleration), which is exactly why so many Unit 2 problems combine kinetic friction with Newton's second law and kinematics. Because friction is a contact interaction between two objects, Newton's third law applies too. The floor pushes backward on the sliding block, and the block pushes forward on the floor with equal magnitude.
Kinetic friction lives in Topic 2.3: Contact Forces in Unit 2: Force and Translational Dynamics, and it supports learning objective 2.3.A, which asks you to describe interactions between two objects using Newton's third law and paired forces. It's one of the contact forces you have to draw correctly on every free-body diagram, and getting its direction right (opposite the sliding, parallel to the surface) is half the battle. Kinetic friction also shows up far beyond Unit 2. It's the classic nonconservative force that removes mechanical energy from a system, so it reappears whenever energy conservation breaks down in later units. If a block slides to a stop, kinetic friction is almost always the reason, and the AP exam loves asking you to account for that missing energy.
Keep studying AP Physics 1 Unit 2
Static Friction (Unit 2)
Static and kinetic friction are two phases of the same interaction. Static friction holds an object in place and adjusts its strength up to a maximum; kinetic friction kicks in the instant sliding starts and stays constant. For most surface pairs μk is less than μs, which is why it takes more force to start a box moving than to keep it moving.
Normal Force (Unit 2)
Kinetic friction is directly proportional to the normal force, so anything that changes N changes friction. On an incline, N = mg cos θ, not mg, and exam questions love to catch people who forget that. Push down on a sliding object and friction grows; lift up on it and friction shrinks.
Newton's Third Law Force Pairs (Unit 2)
Friction is an interaction between two objects, so it comes in pairs (LO 2.3.A). The ground exerts friction backward on a sliding block, and the block exerts an equal-magnitude friction force forward on the ground. Stacked-block problems use this constantly. The friction the top block feels from the bottom block has a third-law partner acting on the bottom block.
Energy Dissipation by Nonconservative Forces (Unit 3)
Kinetic friction is the textbook nonconservative force. The work it does converts mechanical energy into thermal energy, so a block sliding down a rough ramp arrives slower than energy conservation alone would predict. When an energy bar chart doesn't balance, kinetic friction is usually where the energy went.
Kinetic friction is a workhorse in both multiple choice and FRQs. In MCQs, expect free-body diagram questions (which way does friction point on an incline?), Newton's second law calculations using f = μkN, and conceptual traps like whether friction depends on speed (it doesn't) or surface area (it doesn't, in this model). On FRQs, friction often anchors experimental design questions. The 2017 long FRQ Q2 used it, and a recent FRQ had students release a block down a curved ramp and analyze its sliding motion to investigate friction experimentally. You should be ready to design a procedure to measure μk, draw correctly labeled free-body diagrams, derive expressions like a = μk g for a block decelerating on a horizontal surface, and explain in words why the block slows down using Newton's laws or energy reasoning.
Static friction acts when surfaces are NOT sliding past each other; kinetic friction acts when they ARE. Static friction is adjustable, matching the applied force up to a maximum of μsN, while kinetic friction is a fixed value, μkN, no matter how fast the object slides. The sneaky case is a rolling wheel that isn't skidding. Its contact point isn't sliding, so that's static friction, not kinetic. Kinetic friction only applies when there's actual relative sliding between the surfaces.
Kinetic friction acts only when two surfaces are actually sliding past each other, and it always points opposite the direction of sliding.
Its magnitude is f = μkN, where μk is the coefficient of kinetic friction and N is the normal force, which is mg cos θ on an incline rather than mg.
Kinetic friction is constant during sliding; it does not depend on the object's speed or the contact area in the AP model.
For most surfaces μk is less than μs, so it takes less force to keep an object sliding than to start it sliding.
Friction is a Newton's third law interaction, so the surface pushes on the object and the object pushes back on the surface with equal magnitude (LO 2.3.A).
Kinetic friction is a nonconservative force that converts mechanical energy into thermal energy, which is why sliding objects lose energy and stop.
Kinetic friction is the contact force that opposes the sliding motion of an object moving across a surface. Its magnitude is f = μkN, where μk is the coefficient of kinetic friction and N is the normal force, and it appears in Topic 2.3 (Contact Forces) of Unit 2.
No. In the AP Physics 1 model, kinetic friction is constant, equal to μkN, regardless of the object's speed. A block sliding at 1 m/s and 10 m/s on the same surface feels the same friction force, which means it decelerates at a constant rate.
Static friction acts on objects that are not sliding and adjusts its strength up to a maximum of μsN, while kinetic friction acts on objects that are sliding and has a fixed value of μkN. Since μk is usually less than μs, friction drops slightly the moment an object breaks loose and starts moving.
Not if the wheel rolls without slipping. The contact point of a rolling wheel is momentarily at rest relative to the ground, so static friction acts there. Kinetic friction only takes over if the wheel skids, like during a hard brake.
A common design is to slide a block across a surface, measure its deceleration (from motion sensors, video, or kinematics), and use a = μk g on a horizontal surface to solve for μk. Friction experimental-design FRQs like this have appeared on released exams, including a setup where a block slides down a ramp and across a surface.