The opposing force that resists motion when two surfaces come in contact is called friction. Friction is a force that opposes the relative motion of two objects in contact with each other. It is caused by the interaction of microscopic irregularities on the surfaces of the objects. When two objects are in contact, the irregularities on their surfaces interlock, creating a resistance to motion. The amount of friction depends on the nature of the surfaces in contact, the force pressing the objects together, and the relative humidity of the environment.