Specific weight
{{Short description|Weight per unit volume of a material}}
{{Distinguish|Specific gravity|Specific density|Specific force}}
The specific weight, also known as the unit weight (symbol {{mvar|γ}}, the Greek letter gamma), is a volume-specific quantity defined as the weight W divided by the volume V of a material:
Equivalently, it may also be formulated as the product of density, {{mvar|ρ}}, and gravity acceleration, {{mvar|g}}:
Its unit of measurement in the International System of Units (SI) is newton per cubic metre (N/m3), with base units of kg{{sdot}}m−2{{sdot}}s−2.
A commonly used value is the specific weight of water on Earth at {{convert|4|C|abbr=on}}, which is {{convert|9.807|kN/m3|disp=or|lk=on}}.National Council of Examiners for Engineering and Surveying (2005). Fundamentals of Engineering Supplied-Reference Handbook (7th ed.). {{ISBN|1-932613-00-5}}.
Discussion
The density of a material is defined as mass divided by volume, typically expressed in units of kg/m3.
Unlike density, specific weight is not a fixed property of a material, as it depends on the value of the gravitational acceleration, which varies with location (e.g., Earth's gravity).
For simplicity, the standard gravity (a constant) is often assumed, usually taken as {{val|9.81|u=m|up=s2}}.
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 (e.g., N/m3 or lbf/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:
{{glossary}}
{{term|Moist unit weight}}
{{defn|The unit weight of a soil when void spaces of the soil contain both water and air.
where
- {{mvar|γ}} is the moist unit weight of the material
- {{math|γw}} is the unit weight of water
- {{mvar|w}} is the moisture content of the material
- {{math|Gs}} is the specific gravity of the solid
- {{mvar|e}} is the void ratio
}}
{{term|Dry unit weight}}
{{defn|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:
where
- {{mvar|γ}} is the moist unit weight of the material
- {{math|γd}} is the dry unit weight of the material
- {{math|γw}} is the unit weight of water
- {{mvar|w}} is the moisture content of the material
- {{math|Gs}} is the specific gravity of the solid
- {{mvar|e}} is the void ratio
}}
{{term|Saturated unit weight}}
{{defn|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:
where
- {{math|γs}} is the saturated unit weight of the material
- {{math|γw}} is the unit weight of water
- {{math|Gs}} is the specific gravity of the solid
- {{mvar|e}} is the void ratioDas, Braja M. (2007). Principles of Geotechnical Engineering. Canada: Chris Carson. {{ISBN|0-495-07316-4}}.
}}
{{term|Submerged unit weight}}
{{defn|The difference between the saturated unit weight and the unit weight of water.The Transtec Group, Inc. (2012). Basic Definitions and Terminology of Soils. [http://www.intelligentcompaction.com/downloads/IC_RelatedDocs/SoilCmpct_Basic%20definitions%20of%20Soils.pdf] (Page viewed December 7, 2012 It is often used in the calculation of the effective stress in a soil.
The formula for submerged unit weight is:
where
- {{math|γ′}} is the submerged unit weight of the material
- {{math|γs}} is the saturated unit weight of the material
- {{math|γw}} is the unit weight of water
}}
{{glossary end}}
=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
{{See also|Water (molecule)#Density of water and ice|l1=Water density}}
| class="wikitable" style="text-align:center"
|+ Specific weight of water at standard sea-level atmospheric pressure (English units) Finnemore, J. E. (2002). Fluid Mechanics with Engineering Applications. New York: McGraw-Hill. {{ISBN|0-07-243202-0}}. | |
| scope="col" | Temperature(°F)
! scope="col" | Specific weight (lbf/ft3) | |
|---|---|
| 32 | 62.42 |
| 40 | 62.43 |
| 50 | 62.41 |
| 60 | 62.37 |
| 70 | 62.30 |
| 80 | 62.22 |
| 90 | 62.11 |
| 100 | 62.00 |
| 110 | 61.86 |
| 120 | 61.71 |
| 130 | 61.55 |
| 140 | 61.38 |
| 150 | 61.20 |
| 160 | 61.00 |
| 170 | 60.80 |
| 180 | 60.58 |
| 190 | 60.36 |
| 200 | 60.12 |
| 212 | 59.83 |
Specific weight of air
{{Main|Density of air}}
References
{{Reflist}}
External links
- [https://www.ajdesigner.com/phpspecificgravity/specific_gravity_equation_submerged_water_weight_loss.php Submerged weight calculator]
- [http://www.fxsolver.com/solve/share/qtmx1x0eEynB9VGZDGMBLA==/ Specific weight calculator]
- http://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.html
- http://www.themeter.net/pesi-spec_e.htm
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