"Friction may be defined as a force of resistance acting on a body which prevents or retards slipping of the body relative to a second body or surface with which it is in contact. This force always acts tangent to the surface at points of contact with other bodies and is directed so as to oppose the possible or existing motion of the body relative to these points. In general, two types of friction can occur between surfaces. Fluid friction exists when the contacting surfaces are separated by a film of fluid (gas or liquid). The nature of fluid friction is studied in fluid mechanics since it depends upon knowledge of the velocity of the fluid and the fluid's ability to resist shear force. In this book only the effects of dry friction will be presented. This type of friction is often called Coulomb friction since its characteristics were studied extensively by C. A. Coulomb in 1781. Specifically dry friction occurs between the contacting surfaces of bodies in the absence of a lubricating fluid." p 377

DEFORMABLE
ABLE TO CHANGE SHAPE

DRY FRICTION

FORCE
A PUSH OR A PULL

FORCE

FRICTION
A FORCE OF ABRASION OR FLUID DRAG

NONRIGID

NORMAL
IN MECHANICS, NORMAL OFTEN MEANS PERPENDICULAR OR AT RIGHT ANGLES TO A SURFACE OR A DIRECTION. NORMAL CAN ALSO MEAN NOMINAL OR AVERAGE.

NORMAL FORCE
A FORCE THAT IS PERPENDICULAR TO A SURFACE OR THE DIRECTION OF MOTION OF A SYSTEM.

TANGENT

TANGENT TO THE CONTACTING SURFACE