The static friction force must be overcome by an applied force before an object can move. The maximum possible friction force between two surfaces before sliding begins is the product of the coefficient of static friction and the normal force: formula_14. When there is no sliding occurring, the friction force can have any value from zero up to formula_15. Any force smaller than formula_15 attempting to slide one surface over the other is opposed by a frictional force of equal magnitude and opposite direction. Any force larger than formula_15 overcomes the force of static friction and causes sliding to occur. The instant sliding occurs, static friction is no longer applicable—the friction between the two surfaces is then called kinetic friction. However, an apparent static friction can be observed even in the case when the true static friction is zero.