Friction
Friction is a contact force that opposes the relative motion or tendency of motion between two surfaces in contact. It arises from microscopic interactions between surface irregularities and is directly proportional to the normal force through the coefficient of friction. Friction is essential in everyday life — it allows us to walk, cars to brake, and objects to remain stationary on inclined surfaces — but it also causes energy loss as heat in machines.
Key Formula
f = μ × N
LaTeX: f = \mu N
| Symbol | Meaning | Unit |
|---|---|---|
| f | Frictional force | Newton (N) |
| μ | Coefficient of friction (dimensionless, depends on surfaces) | dimensionless |
| N | Normal force between the surfaces | Newton (N) |
Worked Example
Problem
A 15 kg crate is sliding across a concrete floor. The coefficient of kinetic friction between the crate and floor is 0.4. What is the frictional force? Use g = 9.8 m/s².
Solution
First find the normal force: N = m × g = 15 × 9.8 = 147 N. Then f = μ × N = 0.4 × 147 = 58.8 N.
Answer
The frictional force acting on the crate is 58.8 N, opposing its direction of motion.
Typical Coefficients of Friction for Common Surface Pairs
| Surfaces in Contact | Static Friction (μs) | Kinetic Friction (μk) | Application |
|---|---|---|---|
| Rubber on dry concrete | 0.6 – 0.8 | 0.5 – 0.7 | Car tyres braking on road |
| Steel on steel | 0.5 – 0.8 | 0.4 – 0.6 | Machinery and rails |
| Wood on wood | 0.25 – 0.5 | 0.2 – 0.4 | Furniture on floors |
| Ice on ice | 0.03 – 0.05 | 0.02 – 0.04 | Ice skating, glaciers |
| Teflon on Teflon | 0.04 | 0.04 | Non-stick cookware |
| Rubber on wet concrete | 0.45 – 0.6 | 0.35 – 0.5 | Wet road driving |
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.
Newton's Second Law
Newton's Second Law of Motion states that the net force acting on an object equals the product of its mass and acceleration. It is the most quantitative of the three laws and provides the mathematical relationship between force, mass, and motion. This law is used in virtually every engineering and physics calculation involving dynamics, from designing car brakes to launching spacecraft.
From Latin 'frictio' meaning rubbing — from 'fricare' (to rub). The systematic study of friction began with Leonardo da Vinci in the 15th century. Guillaume Amontons rediscovered the proportional laws in 1699, and Charles-Augustin de Coulomb refined them in 1781.