In other words, equilibrium exists whenever all the external forces are in balance. Weight is the force with which gravity pulls on an object. An object with a mass of 1 Kg weighs 9.8 Newtons at Earth's surface. Friction is the surface force existing between objects in contact that acts to oppose the sliding motion, or attempted motion, of one surface over another surface. The force of friction has a direction that is tangent to the surfaces in contact.
Static friction is the force that acts to oppose sliding motion when objects are at rest.
The magnitude of the force of static friction is always equal to the magnitude of the applied force attempting to cause motion. Normal force is the force pressing two surfaces together. The normal force has a direction that is perpendicular to the surfaces in contact. The coefficient of static friction is the ratio of the maximum force of static friction to the normal force. The coefficient of kinetic friction is the ratio of the force of kinetic friction to the normal force.