[Encyclios]

Forces associated with solids

• Normal (N, F_n) – The force between two solids in contact that prevents them from occupying the same space. The normal force is directed perpendicular to the surface. A “normal” in mathematics is a line perpendicular to a planar curve or surface; thus the name “normal force”.
• Friction (f, F_f) – The force between solids in contact that resists their sliding across one another. Friction is directed opposite the direction of relative motion or the intended direction of motion of either of the surfaces.
• Tension (T, F_t) – The force exerted by an object being pulled upon from opposite ends like a string, rope, cable, chain, etc. Tension is directed along the axis of the object. (Although normally associated with solids, liquids and gases can also be said exert tension in some circumstances.)
• Elasticity (F_e, F_s) – The force exerted by an object under deformation (typically tension or compression) that will return to its original shape when released like a spring or rubber band. Elasticity, like tension, is directed along an axis (although there are exceptions to this rule).