# Coefficient of Friction – Meaning, Calculation, Values

#### Engineering

Coefficient of friction is a dimensionless number that represents the ratio between the friction force and normal force acting on a body. In this article, you will learn the meaning of friction coefficient, how to calculate it, values for common objects, and specific use cases of friction coefficient.

## What Does Friction Coefficient Mean?

Coefficient of friction (COF) depicts the relationship between two surfaces in contact and the normal reaction between them. As a result, its value varies according to the materials in contact. Although the coefficient values are usually between 0 and 1, they could be greater than 1 on some occasions.

A value of 0 means there is no friction between the contact surfaces, which is rare, but possible with superfluidity. A value of 1 means that the frictional force and normal force are equal. But if the frictional force is the predominant force between the materials, the coefficient value exceeds 1.

Generally, the coefficient possesses different values for static friction and kinetic/dynamic friction. The static friction coefficient is the value when there is no relative motion between the objects. The kinetic counterpart is the coefficient when one or both objects are moving.

## How to Calculate Coefficient of Friction

Calculating the coefficient of friction is essential in obtaining the right surface combinations per application. In engineering, a typical goal is to select the material combination that minimizes friction, due to its detrimental effects including energy losses and wear. To find the coefficient value (μ), take the ratio of the frictional force (Ff) and the normal force (N). This is the same formula for calculating the coefficient whether it is static (μs) or kinetic (μk). The difference will be that the frictional force should correspond with the type of coefficient being calculated. Normal force remains the same whether the objects are static or in motion.

## COF Values for Common Materials

It is common practice in engineering to empirically determine and document frequently used values for easy and quick reference. The coefficient of friction is one of these values. Apart from varying according to the nature of the materials in contact, the surface condition also influences the COF value.

From the formula, a value above 1 implies that the force that results in relative motion is greater than the normal force. A value of 0 means that there is no friction between the contact surfaces. This is rare, although materials such as Teflon have a coefficient as low as 0.04. The values from the table show that selecting an appropriate material combination and maintaining the surface condition is key to meeting the frictional needs of an application.

## Specific Use Cases

This section focuses on specific use cases of the friction coefficient. It also entails how to influence the coefficient value for a given material combination.

### Coefficient of Friction for Steel on Steel

Generally, the friction coefficient for greasy steel on a steel surface is 0.05 and 0.1 for the kinetic and static values respectively. As for a dry surface, the kinetic and static values are about 0.4 and 0.6 respectively.