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Specific weight

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The specific weight (also known as the unit weight) is the weight per unit volume of a material. The symbol of specific weight is γ (the Greek letter Gamma).

Contents

A commonly used value is the specific weight of water on Earth at 5°C which is 9.807 kN/m3 or 62.43 lbf/ft3.

The terms specific gravity, and less often specific weight, are also used for relative density.

General formula

γ = ρ g

where

γ is the specific weight of the material (weight per unit volume, typically N/m3 units) ρ is the density of the material (mass per unit volume, typically kg/m3) g is acceleration due to gravity (rate of change of velocity, given in m/s2, and on Earth usually given as 9.81 m/s2)

Changes of specific weight

Unlike density, specific weight is not absolute. It depends upon the value of the gravitational acceleration, which varies with location. A significant influence upon the value of specific gravity is the temperature of the material. 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.

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 lb/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, which is the unit weight of a soil when void spaces of the soil contain both water and air.
    • γ = ( 1 + w ) G s γ w 1 + e

    where

    γ is the moist unit weight of the material γ w 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, which is 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:

    γ d = G s γ w 1 + e = γ 1 + w

    where

    γ is the moist unit weight of the material γ d is the dry unit weight of the material γ w 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, which is 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:

    γ s = ( G s + e ) γ w 1 + e

    where

    γ s is the saturated unit weight of the material γ w 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
  • Submerged unit weight, which is defined as 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:

    γ ′ = γ s − γ w

    where

    γ ′ is the submerged unit weight of the material γ s is the saturated unit weight of the material γ w is the unit weight of water

    Mechanical engineering

    Specific weight can be used in mechanical engineering to determine the weight of a structure designed to carry certain loads while remaining intact and remaining within limits regarding deformation.

    References

    Specific weight Wikipedia
    (Text) CC BY-SA


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