In physics, density is the ratio of mass to volume. Any solid object that’s less dense than water floats. Density is an important property of a fluid because mass is continuously distributed throughout a fluid; the static forces and motions within the fluid depend on the concentration of mass (density) rather than the fluid’s overall mass.
Density is mass (m) divided by volume (V), so here’s the formula for density:
In the MKS (Meter-Kilogram-Second) system, the units are kilograms per cubic meter, or kg/m3.
Say you have a whopper diamond with a volume of 0.0500 cubic meters (that’s a cube that’s about 1 foot on each side, so it’s truly a whopper). You measure its mass as 176.0 kilograms. So what’s its density?
Plugging in the numbers and doing the calculations gives you your answer:
So the density of diamond is 3,520 kg/m3. That’s pretty dense.
You can see a sample of the densities of common materials in the table. Note that ice is less dense than water, so ice floats. Generally, solids and gases expand with temperature and therefore become less dense. This table includes the density of water at 4°C as a reference point because the density of water varies with temperature. The densities of the gases generally have a stronger dependence on temperature than the solids do, though.
| Substance | Density (kg/m3) |
|---|---|
| Gold (near room temperature) | 19,300 |
| Mercury (near room temperature) | 13,600 |
| Silver (near room temperature) | 10,500 |
| Copper (near room temperature) | 8,890 |
| Diamond (near room temperature) | 3,520 |
| Aluminum (near room temperature) | 2,700 |
| Blood (near body temperature) | 1,060 |
| Water (4 degrees Celsius) | 1,000 |
| Ice (0 degrees Celsius) | 917 |
| Oxygen (at 0 degrees Celsius, 101.325 kPa) | 1.43 |
| Helium (at 0 degrees Celsius, 101.325 kPa) | 0.179 |