ABSTRACT
Solid are continua—their densities may be mathematically defined. Fluids—gases and liquids—may also satisfy this definition, and so these concepts of stress and strain also apply to them. Fluids are usually defined, and distinguished from solids, as materials that deform continuously under shear stress. This is true no matter how small the applied shear stress is. Also, when normal stress is applied—when a fluid is squeezed in one direction—the fluid flows in the other two directions. The chapter aims to develop a way to talk about the kinematics of deformation of a fluid, this time using strain rate rather than strain as we did for solids. A fluid’s viscosity can be thought of as a measure of how well the fluid flows. Water and maple syrup, for example, flow differently, at different rates; the difference is reflected in their viscosities. Newton’s laws of motion apply to fluids, just as they do to solids.
