In the mechanical energy balance equation, which symbol represents fluid density?

• A. Temperature
• B. Fluid density
• C. Pressure
• D. Velocity

Answer: (B)

In the mechanical energy balance, the symbol for fluid density is the Greek letter rho, representing mass per unit volume. This property is essential because energy terms in the balance are tied to mass and volume relationships. Density lets you convert volumetric flow rate to mass flow rate via the relation mass flow = density × volumetric flow rate, and it appears in energy expressions per unit volume, such as kinetic energy per volume (½ ρ v^2) and potential energy per volume (ρ g h). The other options describe different properties—temperature relates to thermal energy, pressure is a force per area, and velocity is the fluid’s speed—so none of them denote density.