A contact force is a force between two objects (or an object and a surface) that are in contact with each other. This is distinct from a non-contact force, or action-at-a-distance force, such as gravity or magnetic attraction/repulsion. Contact forces always exist in equal and opposite pairs, by Newton's third law.
A contact force has two components. The part of the force that lies within the plane of contact is friction, which must be overcome for the two objects to slide relative to one another along that plane. The part of the force that is perpendicular to the plane of contact is called the normal force. The maximum possible frictional forces,
is proportional to the normal contact force
and the constant of proportionality is denoted by
Thus
If
and there is no movement then it is called limiting friction. If the two objects are in relative motion then
Strictly speaking, contact forces are only a useful simplification for use in classical mechanics. Everyday objects on Earth do not actually touch each other; rather contact forces are the result of the interactions of the electrons at or near the surfaces of the objects (exchange interaction).