Friction is the force that opposes the motion of a object when it is moving while in contact with a surface. It is proportional to the Normal reaction force of the surface on that object. If the object is moving, the force due to friction is called the Kinetic Force of Friction and can be calculated by
Where is the normal reaction force and is called the kinetic coefficient of friction.
If the object is at rest while in contact with a surface, the force due to friction is called the Static Force of Friction and can be calculated by
Note that in this case, is the maximum force due to friction. The magnitude of the static force of friction is usually only as much as is needed to balance out any other external forces in order to keep the object at rest. If the external force become greater than the maximum static force due to friction i.e , the object will start moving.