in materials science and engineering, one is often interested in understanding the forces, or stresses, involved in the deformation of a material. for instance, if the material were a simple spring, the answer would be given by hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium. stresses which can be attributed to the deformation of a material from some rest state are called elastic stresses. in other materials, stresses are present which can be attributed to the rate of change of the deformation over time. these are called viscous stresses. for instance, in a fluid such as water the stresses which arise from shearing the fluid do not depend on the "distance" the fluid has been sheared; rather, they depend on how "quickly" the shearing occurs.