Specific weight


The specific weight, also known as the unit weight, is the weight per unit volume of a material.
A commonly used value is the specific weight of water on Earth at 4°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. A common symbol for specific weight is γ, the Greek letter Gamma.

Definition

The specific weight, γ, of a material is defined as the product of its density,, and the standard gravity, g:
The density of the material is defined as mass per unit volume, typically measured in kg/m3. The standard gravity is acceleration due to gravity, usually given in m/s2, and on Earth usually taken as 9.81 m/s2.
Unlike density, specific weight is not a fixed property of a material. It depends on the value of the gravitational acceleration, which varies with location. 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.

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. 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:
where
The formula for dry unit weight is:
where
The formula for saturated unit weight is:
where
The formula for submerged unit weight is:
where

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.

Specific weight of water

Specific weight of air