Pressure - Density

Pressure (symbol: P) 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.

Formula

Mathematically:

P = F / A where: P is the pressure, F is the normal force, A is the area.

Density

Density is defined as mass per unit volume.

Mathematically:

ρ = m/v

ρ where: (rho) is the density, m is the mass, V is the volume. Different materials usually have different densities, so density is an important concept regarding buoyancy, metal purity and packaging. In some cases density is expressed as the dimensionless quantities specific gravity (SG) or relative density (RD), in which case it is expressed in multiples of the density of some other standard material, usually water or air/gas.

Common units

The SI unit for density is: kilograms per cubic metre (kg/m³) Densities using the following metric units all have exactly the same numerical value, one thousandth of the value in (kg/m³). Liquid water has a density of about 1 kg/dm³, making any of these SI units numerically convenient to use as most solids and liquids have densities between 0.1 and 20 kg/dm³

• kilograms per cubic decimeter (kg/dm³)
• grams per cubic centimeter (g/cc, gm/cc or g/cm³)
• megagrams per cubic meter (Mg/m³)

Liters and metric tons are not part of the SI, but are acceptable for use with it. Since 1 L = 1 dm³, we also have these of the same size:

• kilograms per litre (kg/L)
• grams per millilitre (g/mL)
• metric tons per cubic metre (t/m³)

In U.S. customary units density can be stated in:

• Avoirdupois ounces per cubic inch (oz/cu in)
• Avoirdupois pounds per cubic inch (lb/cu in)
• pounds per cubic foot (lb/cu ft)
• pounds per cubic yard (lb/cu yd)
• pounds per U.S. liquid gallon (lb/gal)
• pounds per U.S. bushel (lb/bu)
• slugs per cubic foot.

In principle there are Imperial units different from the above as the Imperial gallon and bushel differ from the U.S. units, but in practice they are no longer used, though found in older documents. The density of precious metals could conceivably be based on Troy ounces and pounds, a possible cause of confusion.