mechanical engineering

Density
Density is defined as an objects mass per unit volume. Mass is a property.

• Mass and Weight - the Difference! - What is weight and what is mass? An explanation of the difference between weight and mass.

The density can be expressed as
ρ = m / V = 1 / vg (1)
where
ρ = density (kg/m3)
m = mass (kg)
V = volume (m3)
vg = specific volume (m3/kg)

The SI units for density are kg/m3. The imperial (BG) units are lb/ft3 (slugs/ft3). While people often use pounds per cubic foot as a measure of density in the U.S., pounds are really a measure of force, not mass. Slugs are the correct measure of mass. You can multiply slugs by 32.2 for a rough value in pounds.

• Unit converter for other units

The higher the density, the tighter the particles are packed inside the substance. Density is a physical
property constant at a given temperature and density can help to identify a substance.

• Densities and material properties for common materials

Relative Density
Relative density of a substance is the ratio of the substance to the density of water, i.e.

Example - Use the Density to Identify the Material:
An unknown liquid substance has a mass of 18.5 g and occupies a volume of 23.4 ml. (milliliter).

The density can be calculated as
ρ = [18.5 (g) / 1000 (g/kg)] / [23.4 (ml) / 1000 (ml/l) 1000 (l/m3) ]
= 18.5 10-3 (kg) / 23.4 10-6 (m3)
= 790 kg/m3

If we look up densities of some common substances, we can find that ethyl alcohol, or ethanol, has a density of 790 kg/m3. Our unknown liquid may likely be ethyl alcohol!

Example - Use Density to Calculate the Mass of a Volume
The density of titanium is 4507 kg/m3 . Calculate the mass of 0.17 m3 titanium!
m = 0.17 (m3) 4507 (kg/m3)
= 766.2 kg

Specific Weight
Specific Weight is defined as weight per unit volume. Weight is a force.
* Mass and Weight - the difference! - What is weight and what is mass? An explanation of the difference between weight and mass.
Specific Weight can be expressed as

γ = ρ g (2)
where
γ = specific weight (N/m3)
ρ = density (kg/m3)
g = acceleration of gravity (m/s2)

The SI-units of specific weight are N/m3. The imperial units are lb/ft3. The local acceleration g is under normal conditions 9.807 m/s2 in SI-units and 32.174 ft/s2 in imperial units.

Example - Specific Weight Water
Specific weight for water at 39 oF (4 oC) is 62.4 lb/ft3 (9.81 kN/m3) in imperial units. Specific weight in SI units can be calculates like
γ = 1000 kg/m3 9.81 m/s2
= 9810 N/m3
= 9.81 kN/m3