The equations of motion for a viscous fluid which may be written d V/dt = -(1/ρ)∇p + F + ν ∇²V + (¹/₃)ν ∇(∇⋅V), where p is the pressure, ρ the density, F the total external force per unit mass, V the fluid velocity, and ν the kinematic viscosity; for an incompressible fluid, the term in ∇⋅V (divergence) vanishes, and the effects of viscosity then play a role analogous to that of temperature in thermal conduction and to that of density in simple diffusion.