Mass Density Simply Explained
The mass density \( \rho \) is a physical quantity that describes how much mass \( \class{brown}{m} \) is contained in a certain volume \( V \) of a material. The mass density therefore indicates mass per volume:
The unit of mass density is kilogram per cubic meter: \( \mathrm{kg}/\mathrm{m}^3\).
Mass density provides information on how materials behave under different conditions. Materials with different mass densities have different physical properties. For example, a piece of wood floats on water because its mass density is lower than that of water. Metals usually sink in water because their mass density is greater than that of water.
Mass density is also important in technical applications, for example when characterizing materials or calculating buoyancy. The following table contains various materials and their mass density:
| Material | Mass density in kg/m³ |
|---|---|
| Aluminium | 2699 |
| Iron (Ferrium) | 7874 |
| Gold (Aurium) | 19300 |
| Calzium | 1550 |
| Magnesium | 1738 |
| Uranium | 19 050 |
How can you tell if jewelry is made of real gold?
Let's assume you have bought some jewelry and want to know whether it is made of gold and is not fake. How can physics help you with this?
First you have to weigh the jewelry and determine its mass \( \class{brown}{m} \). Then you have to find out the volume \( V \) of the jewelry. To do this, you use Archimedes' principle. This states that the amount of water displaced is exactly equal to the volume of the immersed body. So take a measuring cup of water, throw in the jewelry and look at the mililitre scale to see how high the water has risen. Convert the value into cubic meters.
The density \(\rho\) of the jewelry material can now be calculated with the mass and volume using the formula 1
. The gold has a density of \(19 \, 300 \, \mathrm{kg} / \mathrm{m}^3 \). If you also get this value, then the jewelry is made of real gold.