Density
When we talk about density, we're actually talking about mass density, the mass per unit volume:
| ρ |
= |
| m
|  |
| V
|
|
If an object has a lower density than a surrounding fluid, the object floats in the fluid; if the object has a higher density it sinks.
A density is sometimes specified as a specific gravity, which is the ratio of its density to the density of water at 4°C. For instance, aluminum has a specific gravity of 2.7, which means that its 2.7 times more dense than water. The specific gravity of gold is 19.3.
Densities
| Material (or object) | Density (kg/m3) |
|---|
| Interstellar space | 10-20 |
| Air (20°C and 1 atmosphere)
| 1.21 |
| Water (4°C and 1 atmosphere)
| 1000 |
| Iron | 7900 |
| Mercury (the metal) | 13600 |
| Earth (the planet, on average) | 5500 |
| Sun (on average) | 1400 |
| Black hole | 1019 |