Area density
Encyclopedia
The area density of a two-dimensional object is calculated as the mass
per unit area
. The SI
derived unit is: kilogram
per square metre (kg m−2).
It can be calculated as:
or
where
A special type of area density is called columnar density; it is obtained integrating volumetric density
over a vertical column:
where
denotes the vertical coordinate (e.g., height or depth). An example is the Dobson unit
for atmospheric ozone. Columnar density
is closely related to the vertically averaged volumetric density 
as
where
; notice that 
, 
, and 
have units of, e.g., g/m3, g/m2, and m, respectively.
. When studying bodies falling through air, area density is important because resistance depends on area, and gravitational force is dependent on mass.
The Body mass index
is in terms of area density.
Mass
Mass 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 area
Area
Area is a quantity that expresses the extent of a two-dimensional surface or shape in the plane. Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of paint necessary to cover the surface with a single coat...
. The SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
derived unit is: kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
per square metre (kg m−2).
It can be calculated as:
or
where
| ρA | = average area density |
| M | = total mass of the object |
| A | = total area of the object |
| ρ | = average density Density The 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... |
| l | = average thickness of the object |
A special type of area density is called columnar density; it is obtained integrating volumetric density
Density
The 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...
over a vertical column:where
denotes the vertical coordinate (e.g., height or depth). An example is the Dobson unitDobson unit
The Dobson unit is a unit of measurement of atmospheric ozone columnar density, which is dominated by ozone in the stratospheric ozone layer. One Dobson unit refers to a layer of ozone that would be 10 µm thick under standard temperature and pressure. For example, 300 DU of ozone brought...
for atmospheric ozone. Columnar density
is closely related to the vertically averaged volumetric density aswhere
; notice that , , and have units of, e.g., g/m3, g/m2, and m, respectively.Usage
The area density is often used to describe the thickness of paper, e.g. 80 g/m2 is very common. It is also an important quantity for the absorption of radiationAbsorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
. When studying bodies falling through air, area density is important because resistance depends on area, and gravitational force is dependent on mass.
The Body mass index
Body mass index
The body mass index , or Quetelet index, is a heuristic proxy for human body fat based on an individual's weight and height. BMI does not actually measure the percentage of body fat. It was invented between 1830 and 1850 by the Belgian polymath Adolphe Quetelet during the course of developing...
is in terms of area density.