Fluid properties – does specific gravity have units? Evaluate the statement: “Specific gravity has no units.”

Introduction / Context:
Specific gravity (SG) is frequently used in fluid mechanics and material science to compare densities relative to water or another standard. Whether SG has units is a common conceptual check.

Given Data / Assumptions:

• SG is defined as the ratio of a material’s density to the density of a reference (commonly water at 4 °C for liquids/solids, air for gases in some conventions).
• Both densities are measured in the same units.

Concept / Approach:
Because SG is a ratio of two like quantities (density/density), units cancel. Thus, specific gravity is dimensionless. Its numeric value can depend on temperature and, for gases, reference conditions because density varies with these, but SG itself carries no units.

Step-by-Step Reasoning:

Verification / Alternative check:
Specific weight or relative density can also be used; properly defined, they remain dimensionless ratios when like quantities are compared.

Why Other Options Are Wrong:

• Disagree: incorrect; SG is not expressed in kg/m^3 or any other units.
• “Depends on reference temperature” or “local gravity”: these affect the numeric value of the reference density or weight, not the dimensional nature of the ratio.

Common Pitfalls:
Confusing specific gravity (dimensionless) with specific weight γ = ρ g, which does have units (N/m^3).

Final Answer:
Agree