The
specific weight is the
weightIn science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...
per unit
volumeVolume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
of a material. The symbol of specific weight is
γ (the Greek letter
GammaGamma is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. It was derived from the Phoenician letter Gimel . Letters that arose from Gamma include the Roman C and G and the Cyrillic letters Ge Г and Ghe Ґ.-Greek:In Ancient Greek, gamma represented a...
).
A commonly used value is the specific weight of
waterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
on
EarthEarth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
at 5°C which is 62.43
lbfThe pound force is a unit of force in some systems of measurement including English engineering units and British gravitational units.- Definitions :...
/ft
3 or 9807 N/m
3.
The terms
specific gravitySpecific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
, and less often
specific weight, are also used for
relative densityRelative density, or specific gravity, is the ratio of the density of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water...
.
General formula
where
is the specific weight of the material (
weightIn science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...
per unit
volumeVolume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
, typically N/m
3 units)
is the
densityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of the material (
massMass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
per unit
volumeVolume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
, typically kg/m
3)
is
acceleration due to gravityStandard gravity, or standard acceleration due to free fall, usually denoted by g0 or gn, is the nominal acceleration of an object in a vacuum near the surface of the Earth. It is defined as precisely , or about...
(rate of change of
velocityIn physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
, given in m/s
2)
Changes of specific weight
Unlike density, specific weight is not absolute. It depends upon the value of the
gravitational accelerationIn physics, gravitational acceleration is the acceleration on an object caused by gravity. Neglecting friction such as air resistance, all small bodies accelerate in a gravitational field at the same rate relative to the center of mass....
, which varies with location. A significant influence upon the value of specific gravity is the
temperature Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
of the material.
PressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
may also affect values, depending upon the
bulk modulusThe bulk modulus of a substance measures the substance's resistance to uniform compression. It is defined as the pressure increase needed to decrease the volume by a factor of 1/e...
of the material, but generally, at moderate pressures, has a less significant effect than the other factors.
Fluid mechanics
In
fluid mechanicsFluid mechanics is the study of fluids and the forces on them. Fluid mechanics can be divided into fluid statics, the study of fluids at rest; fluid kinematics, the study of fluids in motion; and fluid dynamics, the study of the effect of forces on fluid motion...
, specific weight represents the
forceIn physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
exerted by gravity on a unit volume of a fluid. For this reason, units are expressed as force per unit volume (e.g., lb/ft
3 or N/m
3). Specific weight can be used as a characteristic
property of a fluid.
Soil mechanics
Specific weight is used as a property of soil often used to solve
earthworkEarthworks are engineering works created through the moving or processing of quantities of soil or unformed rock.- Civil engineering use :Typical earthworks include roads, railway beds, causeways, dams, levees, canals, and berms...
problems.
In soil mechanics, specific weight may refer to:
- Moist unit weight, which is the unit weight of a soil when void spaces of the soil contain both water and air.
where
is the moist unit weight of the material
is the unit weight of water
- w is the moisture content of the material
- Gs is the specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
of the solid
- e is the void ratio
- Dry unit weight, which is 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
is the moist unit weight of the material
is the dry unit weight of the material
is the unit weight of water
- w is the moisture content of the material
- Gs is the specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
of the solid
- e is the void ratio
- Saturated unit weight, which is 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
is the saturated unit weight of the material
is the unit weight of water
- w is the moisture content of the material
- Gs is the specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
of the solid
- e is the void ratio
Mechanical engineering
Specific weight can be used in
mechanical engineeringMechanical engineering is a discipline of engineering that applies the principles of physics and materials science for analysis, design, manufacturing, and maintenance of mechanical systems. It is the branch of engineering that involves the production and usage of heat and mechanical power for the...
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
| Temperature(°F) | Specific weight (lb/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 water at standard sea-level atmospheric pressure (English units) |
| Temperature(°C) | Specific weight (kN/m3) |
| 0 |
9.805 |
| 5 |
9.807 |
| 10 |
9.804 |
| 15 |
9.798 |
| 20 |
9.789 |
| 25 |
9.777 |
| 30 |
9.765 |
| 40 |
9.731 |
| 50 |
9.690 |
| 60 |
9.642 |
| 70 |
9.589 |
| 80 |
9.530 |
| 90 |
9.467 |
| 100 |
9.399 |
| Specific weight of water at standard sea-level atmospheric pressure (Metric units) |
Specific weight of air
| Temperature(°F) | Specific Weight (lb/ft3) |
| −40 |
0.0946 |
| −20 |
0.0903 |
| 0 |
0.08637 |
| 10 |
0.08453 |
| 20 |
0.08277 |
| 30 |
0.08108 |
| 40 |
0.07945 |
| 50 |
0.0779 |
| 60 |
0.0764 |
| 70 |
0.07495 |
| 80 |
0.07357 |
| 90 |
0.07223 |
| 100 |
0.07094 |
| 120 |
0.06849 |
| 140 |
0.0662 |
| 160 |
0.06407 |
| 180 |
0.06206 |
| 200 |
0.06018 |
| 250 |
0.05594 |
| Specific weight of air at standard sea-level atmospheric pressure (English units) |
| Temperature(°C) | Specific weight (N/m3) |
| −40 |
14.86 |
| −20 |
13.86 |
| 0 |
12.68 |
| 10 |
12.24 |
| 20 |
11.82 |
| 30 |
11.43 |
| 40 |
11.06 |
| 60 |
10.4 |
| 80 |
9.81 |
| 100 |
9.28 |
| 200 |
7.33 |
| Specific weight of air at standard sea-level atmospheric pressure (Metric units) |
External links
http://www.engineeringtoolbox.com/density-specific-weight-gravity-d_290.html
http://www.themeter.net/pesi-spec_e.htm