Types of Friction: Static, Kinetic, and Rolling

Friction is a contact force between surfaces that opposes relative motion or the tendency to move; static friction acts on surfaces at rest relative to each other, kinetic (sliding) friction acts when they slide, and rolling friction acts on a rolling wheel. In brief: the maximum static friction is f_s,max = μ_sN and kinetic friction is f_k = μ_kN, where N is the normal force and μ is the coefficient of friction. Usually μ_s > μ_k > μ_rolling.

Friction allows us to walk, brake a car and hold objects, but it also wastes energy and causes wear. This guide describes the three types of friction, gives the model with coefficients, explains the ways of increasing or reducing friction, compares the types in a table, works through a crate problem and corrects common mistakes.

What causes friction?

Even surfaces that look smooth are rough at the microscopic level. The tiny peaks (asperities) of the two surfaces touch at a few points, where they may stick together by molecular forces and interlock. To slide the surfaces, these contacts must be broken. The friction force is parallel to the surface and opposite to the (possible) relative motion. In the simple model, it is proportional to the normal force N and almost independent of the apparent area of contact and, for moderate speeds, of the sliding speed.

What is static friction?

Static friction acts when there is no sliding. It adjusts to match the applied force, up to a maximum: f_s ≤ μ_sN. If you push a heavy box gently, the static friction equals your push, and the box stays at rest. When the push exceeds μ_sN, the box begins to slide. Static friction is also what makes walking possible: the foot pushes back on the ground, and the ground exerts a forward static friction force on the foot, as a tire does when it rolls without slipping.

What is kinetic friction?

When the surfaces slide, kinetic friction acts opposite to the sliding: f_k = μ_kN. Its value is usually a little lower than the maximum static friction, which is why it is easier to keep a heavy object moving than to start it moving. Typical coefficients: rubber on dry concrete μ_k about 0.7, wood on wood about 0.2 to 0.4, steel on ice about 0.03 (these depend on the conditions).

What is rolling friction?

When a wheel rolls on a surface, the deformation of the wheel and the surface causes a small resisting force, written as f_r = μ_rN. The coefficient is usually much smaller than the sliding one (for a car tire on asphalt, about 0.01 to 0.02), which is why wheels and ball bearings are used to reduce friction. A rolling wheel does not slide at the point of contact, so there is static friction there, but no energy is lost to sliding.

Types of friction compared

Static, kinetic and rolling friction
Type	When it acts	Formula	Typical size
Static	No sliding yet	f ≤ μ_sN	Largest maximum
Kinetic	Surfaces sliding	f = μ_kN	Smaller than static maximum
Rolling	Wheel rolling	f = μ_rN	Much smaller

The 20 kg crate with μ_s = 0.50 and μ_k = 0.30. While it stays at rest, static friction rises equal to the push, up to a maximum of 98 N. Once the push exceeds that, the crate slides and friction drops to the constant kinetic value of 58.8 N.

Worked example: pushing a crate

A 20 kg crate lies on a floor with μ_s = 0.50 and μ_k = 0.30. Use g = 9.8 m/s².

Normal force: N = mg = 20 × 9.8 = 196 N.
Maximum static friction: 0.50 × 196 = 98 N.
Push of 80 N: this is less than 98 N, so the crate does not move, and the static friction equals 80 N.
Push of 120 N: this is more than 98 N, so the crate slides. Kinetic friction = 0.30 × 196 = 58.8 N.
Acceleration: net force = 120 − 58.8 = 61.2 N, so a = 61.2 ÷ 20 = 3.1 m/s².

How was friction studied?

Leonardo da Vinci studied friction around 1500 and found that it is independent of the area of contact. Guillaume Amontons rediscovered the laws in 1699, and Charles-Augustin de Coulomb extended them in 1785 by distinguishing static and kinetic friction. Tribology, the science of friction, wear and lubrication, was named in 1966.

Which mistakes are common?

Mistake 1: Using μN for static friction at all times

μ_sN is only the maximum. The actual static friction equals the applied force until it reaches the limit.

Mistake 2: Using weight instead of the normal force

On a slope or with a vertical push or pull, N is not equal to mg.

Mistake 3: Thinking that friction always opposes motion

Friction opposes relative sliding. When you walk, friction on your foot points forward, in the direction of your motion.

Mistake 4: Believing that a bigger contact area gives more friction

In the simple model, friction does not depend on the area of contact, though it does for some materials such as racing tires.