# Coefficient of Friction Formula

The following formula is used to calculate the coefficient of friction using the normal force and linear force.

## Coefficient of Friction Definition

A coefficient of friction is defined as a unitless measure of the resistance to movement two opposing surfaces create due to microscopic surface irregularities.

Essentially, at the microscopic level, no surface is complete flat. It may seem that way to the human touch, but as you zoom in on surfaces you can tell that all surfaces of small bumps and ridges. When two surfaces contact each other, these bumps and ridges hit each other and will try to stop the surfaces sliding past one another.

## Example

The following is an example of how to calculate the coefficient of friction between two materials.

1. First, set up a test procedure with one plate of material laying flat on another. Calculate the normal force of the top plate. This is simply equal to the mass times acceleration. For this example we will say that value is 20N.
2. Next, apply a force in the horizontal direction until the top plate moves. Measure this force. For this example, we will say that force was 5N.
3. Finally, calculate the coefficient of friction between the two plates using the formula above. So, u = F / n = 5N/25N = .25.