* Nature of the surfaces in contact. The roughness or smoothness of the surfaces, as well as the materials they are made of, can affect the amount of friction. For example, rough surfaces tend to have more friction than smooth surfaces.
* Normal force. The normal force is the force that presses the two surfaces together. The greater the normal force, the greater the amount of friction.
* Coefficient of friction. The coefficient of friction is a measure of how resistant the two surfaces are to sliding past each other. The higher the coefficient of friction, the greater the amount of friction.
The formula for calculating the amount of friction between two surfaces is:
```
F = μN
```
where:
* F is the force of friction
* μ is the coefficient of friction
* N is the normal force
