What is Specific Weight?
Specific weight is a weight to volume ratio.Formula
γ = P / V
γ is the symbol used for specific weight, P is the weight and V the volume.
According to the international system of units, specific weight unit is N / m³. Note that we use weight and not mass.
Relative Specific Weight
When you want to compare 2 specific weights you can divide each other to determine what is their relative specific weight.
It's usually used to compare the specific weight of something (γ) to the specific weight of water (γH₂O).
γr = γ/γH₂O
Nota: ɣr is the relative specific weight and ɣH₂O is the specific weight of water.Symbols
- γ (Specific weight)
- ρ (Density)
- SG (Specific gravity)
Units
- N / m
- kN / m
- kN / m²
- kgf / m
- lbf / ft³
Calculating the Specific Weight
You need to know the mass (weight) and the volume to calculate the specific weight.The mass is usually given in an exercise or obtained in practice by scales or load cells.
The weight is calculated by multiplying the mass by the acceleration of gravity (g).
The volume depends on the geometry. I will not lengthen this article by showing each case because this can go pretty far the point I'm trying to make here.
When you collect the data above, just divide the weight by the volume and you will have the result.
Example
Calculate the specific weight of a metalic pipe of 6kg and 0,0004 cubic meters of volume.
Solution:
1) Definition: γ =P/V
2) Calculate the weight:
P = m × g = 6 × 9, 8 = 58, 8N
3) Use the formula:
γ = P/V = 58,8/0,0004 = 147.000N/m³
Credits: Andouglas Junior. Visit the link for more exercises.
Converting Density to Specific Weight
We can also calculate the specific weight by multiplying the density of the material (kg / m³) with the gravitational acceleration (m / s²).
γ = ρ.g
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