The specific weight, also known as the unit weight, is the

weight In science and engineering, the weight of an object is the force acting on the object due to gravity. Some standard textbooks define weight as a vector quantity, the gravitational force acting on the object. Others define weight as a scalar qua ...

per unit

volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). ...

of a material. A commonly used value is the specific weight of

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

at , which is .National Council of Examiners for Engineering and Surveying (2005). ''Fundamentals of Engineering Supplied-Reference Handbook'' (7th ed.). . Often a source of confusion is that the terms ''

specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its dens ...

'', and less often ''specific weight'', are also used for

relative density Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...

. A common symbol for specific weight is , the Greek letter

Gamma Gamma (uppercase , lowercase ; ''gámma'') is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter r ...

Definition

The specific weight, , of a material is defined as the product of its

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

, , and the

standard gravity The standard acceleration due to gravity (or standard acceleration of free fall), sometimes abbreviated as standard gravity, usually denoted by or , is the nominal gravitational acceleration of an object in a vacuum near the surface of the Earth. ...

, :

\gamma = \rho \, g

The

of the material is defined as

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different ele ...

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 . Unlike density, specific weight is not a fixed property of a material. It depends on the value of the

gravitational acceleration In physics, gravitational acceleration is the acceleration of an object in free fall within a vacuum (and thus without experiencing drag). This is the steady gain in speed caused exclusively by the force of gravitational attraction. All bodie ...

, which varies with location.

Pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...

may also affect values, depending upon the

bulk modulus The bulk modulus (K or B) of a substance is a measure of how resistant to compression the substance is. It is defined as the ratio of the infinitesimal pressure increase to the resulting ''relative'' decrease of the volume. Other moduli descri ...

of the material, but generally, at moderate pressures, has a less significant effect than the other factors.

Applications

Fluid mechanics

fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics of fluids ( liquids, gases, and plasmas) and the forces on them. It has applications in a wide range of disciplines, including mechanical, aerospace, civil, chemical and ...

, specific weight represents the

force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a ...

exerted by

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

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 Property is a system of rights that gives people legal control of valuable things, and also refers to the valuable things themselves. Depending on the nature of the property, an owner of property may have the right to consume, alter, share, r ...

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:

Civil and mechanical engineering

Specific weight can be used in

civil engineering Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including public works such as roads, bridges, canals, dams, airports, sewa ...

and

mechanical engineering Mechanical engineering is the study of physical machines that may involve force and movement. It is an engineering branch that combines engineering physics and mathematics principles with materials science, to design, analyze, manufacture, ...

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

References

External links

Water Weights & Weight Belts (Arrow Weights Manufacturing)

Specific weight calculator
* http://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.html * http://www.themeter.net/pesi-spec_e.htm {{Authority control Soil mechanics Fluid mechanics Physical chemistry Physical quantities Density