[[Specific Volume and Density|Density]] is related to [[Momentum|momentum]] and [[Energy|energy]] in [[Fluid|fluid]] flows, taking the place of [[Mass|mass]] in Newton’s second law. An increase in density for a fluid flow at the same [[Velocity|speed]] will yield an increase in momentum and energy. Flows can be classified in two ways regarding density: - **Compressible:** Density is a function of location and time: $\rho=\hat{\rho}(x,y,z,t)$ - **Incompressible:** Density is constant throughout the entire [[Velocity Field|flow field]], both spatially and temporally.