Friction is a contact force that resists the relative sliding of two surfaces. It acts parallel to the surfaceof contact and points opposite to the direction the object slides — or, if nothing is sliding yet, opposite to the direction it would slide. Microscopically it comes from the roughness and molecular bonding between two surfaces pressed together.
Two ingredients set the size of friction: how hard the surfaces are pressed together (the normal force F_N) and how rough they are (the coefficient of friction μ, a dimensionless number). More normal force or a rougher pairing means more friction.
Kinetic friction acts whenever two surfaces are actually sliding against each other. It has a fixed magnitude given by:
Here μ_k is the coefficient of kinetic friction and F_N is the normal force. Kinetic friction always opposes the direction of sliding, and — importantly — its size doesn't depend on how fast the object moves. Whether the block slides at 1 m/s or 10 m/s, f_k is the same as long as F_N doesn't change.
A 8 kg crate slides across a level floor with μ_k = 0.25. Find the kinetic friction force acting on it. Use g = 10 m/s².
When a surface is tilted, gravity still pulls straight down — but it now has to be split into two components relative to the incline. This is where the rotated coordinate system from Lesson 2.2 pays off.
Adjust the incline angle and surface properties. Watch how the parallel component of gravity and the friction force compete — and see exactly when the block breaks free.
Critical angle is 26.6° for these surfaces. Below it the block holds; above it the block slides. Try setting θ exactly there — static friction is at its maximum and the block is right on the verge.
A 6 kg block sits on a 30° incline with μₛ = 0.45 and μₖ = 0.28. Does it slide? If so find its acceleration. Use g = 10 m/s².
Static friction acts when the surfaces are not sliding. Its defining feature: it's a variable force. It automatically adjusts to exactly cancel whatever force is trying to start the motion — up to a maximum value. That's why the equation uses ≤:
If you push a heavy box gently, it doesn't move: static friction rises to match your push exactly, so the net force stays zero. Push harder and static friction grows with you. Only when your push exceeds the maximum, μ_s·F_N, does the box finally break loose.
For most surface pairings, the maximum static friction is greaterthan the kinetic friction (μ_s > μ_k). That's why it takes a bigger shove to start a heavy object moving than to keep it moving. The moment it breaks free, friction drops and — if you keep pushing at the same force — the object suddenly accelerates.
A 5 kg block sits on a floor (μ_s = 0.5, μ_k = 0.3, F_N = 50 N). Push harder and harder. Static friction matches your push — until you exceed its maximum of 25 N, and the block breaks free into kinetic friction.
A 5 kg block sits on a floor with μ_s = 0.5 and μ_k = 0.3 (g = 10 m/s²). You push horizontally with 30 N. Does it move? If so, find its acceleration.
A frequent surprise: in the AP Physics 1 model, the friction force does not depend on the contact area. Look at the equations — f = μ·F_N contains only the coefficient and the normal force. Area appears nowhere. A brick sliding on its wide face and the same brick sliding on its narrow end experience the same friction (same weight, same μ).
Normal force (F_N) and the coefficient of friction (μ) — the roughness of the two surfaces.
Contact area, or the speed of sliding (for kinetic friction).