Contact and Non-Contact Forces

Forces can also be classified by whether physical contact between objects is required.

NON-CONTACT forces exist even when the interacting objects are physically separated -- gravitational force, electrostatic force and magnetostatic force are the standard examples; distance alone is enough for these forces to act.

CONTACT forces arise only when two objects are actually touching, and are always electromagnetic in origin, resulting from deformation at the point of contact. There are two common categories: (a) NORMAL REACTION -- when two objects in contact press on each other, each pushes the other away along their common normal direction (e.g. standing on a table: you push the table down, it pushes you up, equal in magnitude, along the same normal line); (b) FRICTION -- the component of the contact force that is parallel (tangential) to the surfaces in contact, which arises whenever there is relative motion, or a tende …