What is a specific weight?
Specific weight, also called unit weight or sometimes weight density, describes the weight of the substance due to its volume. It is often used as a characteristic property of fluids and soils in the fields of fluid mechanics and soil mechanics. The specific weight of any given substance is not constant - it can change based on temperature and pressure.
Gamma Greek letter that looks like "y" usually represents a specific weight in equations. The equation of "gamma = rho*g" is usually used to calculate specific weights. Rho, a Greek letter that looks like a rounded "P" represents the density of the substance - the weight of the substance due to its volume. "G", sometimes called the local constant, represents acceleration due to gravity, which is 32.2 feet per square second (about 9.81 meters per square second) on the Earth's surface.
Standard units of density are slugs on a cubic track (slugs/ft^3) or pounds-Mass on a cubic foot (LBM/FT^3) in the imperial units and kilograms onCubic meter (kg/m^3) in metric units. The "G" constant is measured in footsteps per second per square second (ft/s^2) or meters on a square second (m/s^2). The multiplication of "G" density results in a pound unit on a cubic track (LBF/FT^3) or Newons per cubic meter (n/m^3).
As an example, water in metric units has a density of 1,000 kg/m^3. The multiplication of 9.81 m/s^2 results in a specific weight of 9 810 n/m^3. In the imperial units, the water density is 1.94 slugs/ft^3 and multiplication 32.2 ft/s^2 leads to 62.4 lb/ft^3. However, this calculation is not used when density in LBM is measured. One pound mass is equivalent to one pound force, so if the fabric has a density of 10 lbm/ft^3, it will have a specific weight of 10 lbf/ft^3.
When using with reference to soil, a specific weight is generally referred to as unit weight and phenomena is somewhat different. Two types are usually calculated for soil samplesUnit weight: mass unit weight and dry unit weight. The weight of the volume unit is the unit weight of the sample when the pores in the soil contain air and water. To determine the weight of the dry unit, the laboratory device is used to completely dry the soil sample, so it does not contain any water. The mass of the bulk unit is defined as a total weight divided by a total volume, while the weight of the dry unit is a dry weight divided by a total volume.
density changes based on pressure and temperature and because the specific weight is based on density, it can also change. The density decreases as the temperature increases as the molecules in the substance move further apart. The density increases as the pressure increases as the pressure forces the molecules closer to each other.