FLUID MECHANICS
Properties of fluids : Mass Density, Specific Weight and Specific Gravity
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What is a Fluid?
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A fluid is defined as any substance or material that is inherently capable of flowing under the application of external forces.
From a rigorous scientific and engineering perspective: A fluid is a substance that deforms continuously (flows) when subjected to an applied shear force (stress), no matter how small that mechanical shear force may be. This fundamental property includes both liquids and gases within the broader category of fluid media.
Basic Classification of Materials
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We can broadly classify common matter based on how their constituent molecules are arranged, the relative strength of their internal structural bonds, and their macroscopic behavior under external pressure or force configurations.
| Classification | Molecular / Attraction (Microscopic View) | Shape & Volume (Macroscopic View) | Compressibility State |
| SOLID 🧱 | • Extremely strong bonding forces between adjacent molecules. • Fixed, rigid internal crystal lattice structure. | • Possesses a definite shape. • Possesses a definite volume. | • Completely incompressible under standard working conditions. |
| LIQUID (Fluid) 💧 | • Molecules are spaced slightly further apart. • Slightly weak internal bonding allowing relative movement. | • Mainatins a definite volume. • Exhibits a variable shape (takes the exact form of its container). | • Considered practically incompressible for standard calculations. |
| GAS (Fluid) 💨 | • Very large molecular spacing gaps. • Negligible molecular attraction forces. | • Both shape and volume change dynamically to fully fill the enclosing container volume. | Highly compressible state of matter. |
1. Mass Density (ρ) / Specific Mass
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Definition: Mass Density (ρ) is defined as the ratio of the total mass (M) of a fluid to the total geometric volume (∀) that it occupies. It fundamentally measures the quantity of matter packed into a unit space.
