I am confused on the definition of the "no-slip" boundary condition because of two seemingly contradicting definitions. Definition 1: The no-slip condition for viscous fluid states that at a solid boundary, the fluid will have zero velocity relative to the boundary. Definition 2: The fluid velocity at all liquid–solid boundaries is equal to that of the solid boundary. What is the velocity at a solid boundary if its moving? This would contradict the zero velocity definition. Take the example of a air-liquid-solid system, with air on top, liquid in the middle, and the solid on the bottom. Suppose the bottom plate is pulled with a velocity V, at steady-state, to the right-hand side of the system. What would the boundary condition be and/or what would the velocity and shear stress profile look like? (Cartesian coordinates with y in the "north" direction and x in the "east direction") My guess for the boundary conditions would be that the v=V at y=0 and v=0 at y=[tex]\delta[/tex]. Is this the correct logic?