The coefficient of kinetic friction (μk) is the dimensionless ratio of the kinetic friction force to the normal force between two surfaces sliding past each other, so Fk = μk·FN. It depends only on the two materials in contact and shows up throughout AP Physics 1 Unit 2 (Topics 2.3 and 2.6).
The coefficient of kinetic friction, written μk, tells you how "grippy" two surfaces are while they slide against each other. It's defined by the equation Fk = μk·FN, where Fk is the kinetic friction force and FN is the normal force pressing the surfaces together. Because it's a ratio of two forces, μk has no units. Rubber on dry concrete has a high μk; a hockey puck on ice has a tiny one.
Here's the intuitive version. μk is the exchange rate between "how hard the surfaces are pressed together" and "how hard friction drags back." Double the normal force and you double the kinetic friction force, but μk itself stays the same because it's a property of the surface pair, not the situation. In the AP Physics 1 model, kinetic friction doesn't care how fast the object slides or how much surface area touches. It points opposite the direction of sliding, always.
μk lives in Unit 2: Force and Translational Dynamics, specifically Topic 2.3 (Contact Forces) and Topic 2.6 (Newton's Second Law). Friction is a contact force, and the paired friction forces between two surfaces are a classic Newton's third law interaction (the floor's friction on the box and the box's friction on the floor are equal and opposite, per LO 2.3.A). But the real payoff is Topic 2.6, where μk becomes a workhorse in second law problems. Almost every block-on-incline, box-being-dragged, or stacked-blocks problem needs you to compute Fk = μk·FN before you can find the net force and acceleration. The subtle skill the exam loves is recognizing that FN is not always mg. On an incline it's mg·cos(θ), and if a rope pulls upward at an angle, FN shrinks, so friction shrinks too.
Keep studying AP Physics 1 Unit 2
Coefficient of Static Friction (Unit 2)
These two coefficients describe the same surface pair in different states. μs governs surfaces that aren't sliding yet, μk takes over once sliding starts, and μk is typically smaller. That's why it takes a bigger push to get a heavy box moving than to keep it moving.
Constant Velocity (Unit 2)
An object sliding at constant velocity is the cleanest μk setup on the exam. Zero acceleration means the applied force exactly balances kinetic friction, so you can read μk straight off the force balance. This is the basis of a classic experimental design question.
Newton's Second Law (Unit 2)
μk is rarely the answer by itself. It's the ingredient you need to build Fnet = ma. The standard pipeline is draw the free-body diagram, find FN from the perpendicular direction, compute Fk = μk·FN, then solve for acceleration in the direction of motion.
Kinetic Energy (Unit 3)
Kinetic friction is the main way mechanical energy leaves a system in AP Physics 1. When a block slides distance d, friction removes μk·FN·d of kinetic energy, converting it to thermal energy. This links your Unit 2 force toolkit directly to Unit 3 energy bar charts and stopping-distance problems.
Multiple-choice questions usually hand you μk and make you do something with it. Find the acceleration of a block on a rough incline, compare friction forces when the normal force changes, or rank scenarios by stopping distance. Watch for stems where FN ≠ mg, because that's the most common trap. On the free-response side, μk is a favorite for experimental design. The 2026 FRQ Q3 had students release a block of unknown mass down a curved ramp to investigate friction, exactly the kind of setup where you design a procedure, decide what to measure, and explain how the data leads to μk. Notice the "unknown mass" detail. Mass often cancels out of μk calculations, and explaining why is the kind of reasoning the FRQ rubric rewards.
Static friction (μs) applies before sliding starts and gives a maximum, not a fixed value, since static friction adjusts to match the applied force up to μs·FN. Kinetic friction (μk) applies during sliding and gives one constant value, Fk = μk·FN, no inequality. For a given surface pair, μk is usually less than μs. On the exam, your first move in any friction problem is deciding whether the surfaces are sliding relative to each other. Wrong coefficient means wrong answer from step one.
The coefficient of kinetic friction is defined by Fk = μk·FN, where FN is the normal force, and it has no units because it's a ratio of forces.
μk depends only on the two materials in contact, not on speed, surface area, or how hard you push horizontally.
The normal force is not always mg. On an incline it's mg·cos(θ), and any vertical component of an applied force changes it, which changes the friction force.
μk applies only while surfaces are actually sliding past each other; if they're not sliding yet, you need the coefficient of static friction instead.
Mass often cancels when solving for μk, which is why FRQs can ask you to find μk for a block of unknown mass.
An object dragged at constant velocity is in equilibrium, so the applied force equals μk·FN, making it the go-to setup for measuring μk experimentally.
It's the dimensionless ratio μk = Fk/FN that relates the kinetic friction force on a sliding object to the normal force pressing the surfaces together. It's a property of the two materials in contact and is central to Unit 2 force problems.
Yes, it can. Most everyday surface pairs have μk below 1, but nothing in physics caps it there. Very sticky pairs like rubber on rubber can exceed 1. On the AP exam, though, you'll almost always see values between 0 and 1.
μs applies before sliding and sets a maximum possible friction force (fs ≤ μs·FN), while μk applies during sliding and gives one fixed value (Fk = μk·FN). For the same surfaces, μk is typically smaller than μs, which is why objects lurch forward once they break free.
No. In the AP Physics 1 model, kinetic friction is constant during sliding regardless of speed. It depends only on μk and the normal force, which is what separates it from velocity-dependent forces like air resistance.
No. Any μk value you need will be given in the problem, or the question will ask you to calculate it from measurements. The equation Fk = μk·FN is on the AP Physics 1 equation sheet, so focus on applying it, especially finding the correct normal force.