Consider two relatively flat surfaces that are in contact, such as a block on a tabletop. When there is no relative motion between these two surfaces, the coefficient of initial friction is the ratio of the force applied to initiate relative sliding motion (parallel to the surfaces) to the force due to the load (perpendicular to the surfaces). The coefficient of kinetic friction is the ratio of the applied force to the load in order to maintain sliding motion. The term coefficient of friction usually refers to the coefficient of kinetic friction.