# Coefficient of friction

A coefficient of friction is a value that shows the relationship between the force of friction between two objects and the normal force between the objects. It is a value that is sometimes used in physics to find an object's normal force or frictional force when other methods aren't available.

The coefficient of friction is shown by ${\displaystyle f=\mu F_{n}\,}$. In that equation, ${\displaystyle f}$ is the frictional force, ${\displaystyle \mu }$ is the coefficient of friction, and ${\displaystyle F_{n}\,}$ is the normal force.

${\displaystyle \mu }$ can be two different things. It can be ${\displaystyle \mu _{s}}$, or it can be ${\displaystyle \mu _{k}}$. ${\displaystyle \mu _{s}}$ is the coefficient of static friction. The coefficient of static friction is the friction force between two objects when neither of the objects is moving. ${\displaystyle \mu _{k}}$ is the coefficient of kinetic friction. The coefficient of kinetic friction is the force between two objects when one object is moving, or if two objects are moving against each other.

The coefficient of friction is dimensionless, meaning it does not have any units. It is a scalar, meaning the direction of the force does not affect the physical quantity.

The coefficient of friction depends on the objects that are causing friction. The value is usually between 0 and 1 but can be greater than 1. A value of 0 means there is no friction at all between the objects. This is only theoretically possible. All objects in the real world will have some friction when they touch each other. A value of 1 means the frictional force is equal to the normal force. Some people think that the coefficient of friction can never be more than 1, but this is not true. A coefficient of friction that is more than one just means that friction is stronger than the normal force. An object such as silicone rubber, for example, can have a coefficient of friction much greater than one. The coefficient of friction can also be changed by the mass and speed of the moving object.

The friction force is the force exerted by a surface when an object moves across it - or makes an effort to move across it.

The frictional force can be expressed as

Ff = μ N (1)

where

Ff = frictional force (N, lb)

μ = static (μs) or kinetic (μk) frictional coefficient

N = normal force (N, lb)