When two surfaces slide over one another the tiny bumps push on each other. Friction causes a force on a surface which is in the opposite direction to its motion. Friction can be reduced by the application of lubricants.

What causes frictional force to occur?

Hint: Friction is a resistive force which resists relative motion between surfaces in contact with each other. Friction is caused due to the irregularities of the surfaces in contact. ... When the two surfaces are moved against each other these bonds resist the motion creating friction.

What is frictional force when does it come into play?

The force of friction come into play when one body moves upon another body.