1. Friction is caused by the irregularities on the two surfaces in contact.
2. Even those surface which appear very smooth have a large number of minute irregularities on them.
3. Irregularities on the two surfaces lock into another.
4. When we attempt to move any one surface over the other, we have to apply a force to overcome interlocking.
5. On rough surfaces, there are a larger number of irregularities.
6. So the force of friction is greater if a rough surface is involved.
