The specific weight is defined as the weight per volume unit and is obtained by dividing the weight of a system by the volume it occupies. It is represented by the symbol γ (Greek letter gamma) and in the International System of Units it is measured in N/m3 (newtons per cubic meter).
It is a term related to Relative density, also called specific gravity, since weight is mass times gravity and density is mass divided by volume. In fact, the specific weight can be calculated as the density due to gravity:
Where:
- γ is the specific weight
- w is the weight of the substance
- v is volume
- m is the mass
- g is the acceleration due to gravity
- ρ is the density of the substance
Specific gravity is often used as a property of a fluid and as such it is used in fluid mechanics as a representation of the force exerted by gravity on a volume of a fluid material, hence the units of specific gravity (N/m3) express a force per unit volume. It is also used in soil mechanics, mechanical engineering, and many other fields.
Specific gravity in the SI
From a strict point of view, the specific weight would not fall within the definition of specific property of the International System, since a specific property is defined as property per unit of mass, not volume as is the case of specific gravity.
A specific property is defined as a composite property that is obtained by divide an extensive property by the mass of the system. Extensive properties are those that change when the total mass of the system changes, but specific properties become intensive properties and remain constant with changes in mass.
For example, volume in the International System is measured in cubic meters (m3) and is an extensive property since it depends on the mass. For example, 4 kg of a certain material can occupy 10 m3. The specific volume is obtained by dividing the volume by the mass of the system and represents the volume occupied per unit mass; in this case 10 / 4 = 0.25m3/kg, which means that 1 kg of the material occupies 0.25 m3. The specific volume, unlike the normal volume, becomes an intensive property, since it is the same even if the amount of mass changes: if instead of 4 kg of material we have 8 kg, the system will occupy 20 m3 and 20 / 8 is also equal to 0.25. The volume changes, the specific volume does not.
Specific gravity, being a property related to volume and not to mass directly, does not enter into the definition, although it is a widely used and accepted term.