Specific gravity is the weight of a substance relative to its volume, often used in fluid and soil mechanics. It can change with temperature and pressure. Gamma represents specific gravity in equations, calculated using density and acceleration due to gravity. Units of density are snails/ft^3 or kg/m^3. Specific gravity of water is 9.810 N/m^3 or 62.4 lb/ft^3. Unit weight is used for soil samples, calculated as bulk or dry unit weight. Density changes with pressure and temperature, affecting specific gravity.
Specific gravity, also called unit weight or sometimes weight density, describes the weight of a substance relative to its volume. It is often used as a characteristic property of fluids and soils in the fields of fluid mechanics and soil mechanics, respectively. The specific gravity of a given substance is not constant: it can change according to temperature and pressure.
The lowercase Greek letter gamma, which looks like a “y,” usually represents specific gravity in equations. Typically, the equation “gamma = rho*g” is used to calculate specific gravities. Rho, a Greek letter that looks like a rounded “p,” represents the density of the substance, the mass of the substance relative to its volume. Sometimes called the local constant, “g” represents the acceleration due to gravity, which is 32.2 feet per second squared (about 9.81 meters per second squared) at the Earth’s surface.
The standard units of density are snails per cubic foot (snails/ft^3) or pounds-mass per cubic foot (lbm/ft^3) in imperial units and kilograms per cubic meter (kg/m^3) in metric units. The “g” constant is measured in feet per second squared (ft/s^2) or meters per second squared (m/s^2). Multiplying the density by “g” gives units of pounds-force per cubic foot (lbf/ft^3) or Newtons per cubic meter (N/m^3).
For example, water has a density of 1,000 kg/m^3 in metric units. Multiplying by 9.81 m/s^2 we obtain a specific gravity of 9.810 N/m^3. In imperial units, the density of water is 1.94 slugs/ft^3 and multiplying by 32.2 ft/s^2 gives 62.4 lb/ft^3. However, this calculation is not used when density is measured in pounds. One pound of mass is equivalent to one pound of force, so if a substance has a density of 10 lbf/ft^3, it will have a specific gravity of 10 lbf/ft^3.
When used in reference to soils, specific gravity is usually referred to as unit weight and is calculated slightly differently. Typically, two types of unit weight are calculated for soil samples: bulk unit weight and dry unit weight. Bulk unit weight is the unit weight of the sample when the pores of the soil contain both air and water. To determine unit dry weight, laboratory equipment is used to completely dry a soil sample so that it contains no water. Bulk unit weight is defined as the total weight divided by the total volume, while dry unit weight is the dry weight divided by the total volume.
Density changes with pressure and temperature, and since specific gravity is based on density, it can also change. The density decreases as the temperature increases because the molecules in the substance move further apart. Density increases as pressure increases because pressure forces molecules closer together.
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