Friction is a force that resists the relative motion between two objects or materials. The causes of the resistive force of friction are:
- Molecular Adhesion
- Surface Roughness
- Plowing effect
Adhesion is a molecular force that arises when two materials are brought close to each other.
Surface roughness is a factor in friction when the materials are rough enough to cause serious abrasion.
When one or more of the materials are relatively soft, much of the resistance to movement is caused by deformations or a plowing effect.