The general term for fluid resistance force is damping. It is produced when a fluid layer moves relative to an object. Such is the case, for example, when the block in Figure 1.4-1 slides on a lubricated surface. The fluid’s viscosity produces a shear stress that exerts a resisting force on the block and on the surface. Viscosity is an indication of the “stickness” of the fluid; molasses and oil have greater viscosities than water, for example. This resisting force is called the damping force, and its magnitude depends on the relative velocity between the fluid and the surface. This dependence can be quite complicated, but we usually model it as a linear function of the relative velocity. This approach allows us to obtain equations of motion that are easier to solve, without ignoring the effect of the velocity-dependent damping force.