The friction of a non-moving object is called static friction. It is the force that prevents an object from starting to move when a force is applied to it. Static friction is caused by the interlocking of the surface irregularities of the object and the surface it is resting on. It is greater than or equal to kinetic friction, which is the friction that opposes the motion of an object that is already moving. The coefficient of static friction is the ratio of the force of static friction to the normal force between the object and the surface.