Specific weight
Weight per unit volume of a material
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Discussion
The density of a material is defined as mass divided by volume, typically expressed with the unit kg/m3. Unlike density, specific weight is not a fixed property of a material, as it depends on the value of the gravitational acceleration, which varies with location (e.g., Earth's gravity). In practice, the standard gravity (a constant) is often assumed, usually taken as 9.80665 m/s2.
Pressure may also affect values, depending upon the bulk modulus of the material, but generally, at moderate pressures, has a less significant effect than the other factors.
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Applications
Fluid mechanics
In fluid mechanics, specific weight represents the force exerted by gravity on a unit volume of a fluid. For this reason, units are expressed as force per unit volume (e.g., N/m3 or lbf/ft3). Specific weight can be used as a characteristic property of a fluid.
Soil mechanics
Specific weight is often used as a property of soil to solve earthwork problems.
In soil mechanics, specific weight may refer to:
- Moist unit weight
- The unit weight of a soil when void spaces of the soil contain both water and air. where
- γ is the moist unit weight of the material
- γw is the unit weight of water
- w is the moisture content of the material
- Gs is the specific gravity of the solid
- e is the void ratio
- Dry unit weight
- The unit weight of a soil when all void spaces of the soil are completely filled with air, with no water. The formula for dry unit weight is: where
- γ is the moist unit weight of the material
- γd is the dry unit weight of the material
- γw is the unit weight of water
- w is the moisture content of the material
- Gs is the specific gravity of the solid
- e is the void ratio
- Saturated unit weight
- The unit weight of a soil when all void spaces of the soil are completely filled with water, with no air. The formula for saturated unit weight is: where
- γs is the saturated unit weight of the material
- γw is the unit weight of water
- Gs is the specific gravity of the solid
- e is the void ratio
- Submerged unit weight
- The difference between the saturated unit weight and the unit weight of water. It is often used in the calculation of the effective stress in a soil. The formula for submerged unit weight is: where
- γ′ is the submerged unit weight of the material
- γs is the saturated unit weight of the material
- γw is the unit weight of water
Civil and mechanical engineering
Specific weight can be used in civil engineering and mechanical engineering to determine the weight of a structure designed to carry certain loads while remaining intact and remaining within limits regarding deformation.