Kinetic Friction
Kinetic friction is the resistive force that acts between two surfaces in relative motion, opposing the direction of movement. It arises from microscopic interactions — adhesion, deformation, and surface roughness — between the contacting surfaces. In engineering and everyday life, kinetic friction is exploited in braking systems and must be minimised in bearings and sliding machinery.
Key Formula
f_k = mu_k × N
LaTeX: f_k = \mu_k N
| Symbol | Meaning | Unit |
|---|---|---|
| f_k | Kinetic friction force | N (Newton) |
| \mu_k | Coefficient of kinetic friction (dimensionless) | — |
| N | Normal force acting perpendicular to the surface | N (Newton) |
Worked Example
Problem
A 10 kg wooden block slides across a concrete floor. The coefficient of kinetic friction between wood and concrete is 0.35. Calculate the kinetic friction force acting on the block. (Take g = 9.8 m/s².)
Solution
Step 1 — Find the normal force: Since the surface is horizontal and there is no vertical acceleration, N = mg = 10 × 9.8 = 98 N. Step 2 — Apply the kinetic friction formula: f_k = μ_k × N = 0.35 × 98 = 34.3 N.
Answer
The kinetic friction force is 34.3 N, directed opposite to the block's motion.
Typical coefficients of kinetic friction for common material pairs
| Surface Pair | μ_k (approx.) | Typical Application | Notes |
|---|---|---|---|
| Steel on steel (dry) | 0.57 | Machine slides | Increases with oxidation |
| Wood on concrete | 0.35 | Furniture, floors | Varies with moisture |
| Rubber on asphalt (dry) | 0.67 | Vehicle tyres | Key for braking distance |
| Ice on ice | 0.03 | Winter sports | Very low — near frictionless |
| PTFE on PTFE | 0.04 | Industrial bearings | Lowest solid-solid value |
Interactive Tools
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Related Terms
Normal Force
The normal force is the contact force exerted by a surface on an object, acting perpendicular (normal) to the surface at the point of contact. It is a reaction force that prevents objects from passing through solid surfaces and adjusts in magnitude to balance components of other forces. On a flat horizontal surface, the normal force on a stationary object equals its weight; on an inclined surface, it equals the component of weight perpendicular to the slope.
Static Friction
Static friction is the frictional force that prevents a stationary object from beginning to move when a force is applied to it. It is a self-adjusting force that increases to match the applied force until a maximum threshold is reached; beyond this maximum, the object begins to slide and kinetic friction takes over. The maximum static frictional force is always greater than the kinetic frictional force for the same surfaces, which is why it is harder to start sliding an object than to keep it sliding.
From Latin "kineticus" (relating to motion), derived from Greek "kinētikos" (putting in motion), root "kinein" (to move). The word "friction" comes from Latin "frictio" (rubbing), from "fricare" (to rub). The distinction between static and kinetic friction was formalised by Guillaume Amontons and later Charles-Augustin de Coulomb in the 17th–18th centuries.