ABSTRACT
What is a fluid? I t isn't a solid, but what is a solid? Perhaps it is easier to define these materials in terms of how they respond (i.e., deform or flow) when subjected to an applied force in a specific situation such as the s i m p l e s h e a r situation illustrated in Fig. 3-1 (which is virtually identical to Fig. 1-1 ). We envision the material contained between two infinite parallel plates, the bottom one being fixed and the top one subject to an applied force parallel to the plate, which is free to move in its plane. The material is assumed to adhere to the plates, and its properties can be classified by the way the top plate responds when the force is applied.
