A fluid is anything that can flow. Liquids are certainly fluids, but so are gases.
When we talk about density, we're actually talking about mass density, the mass per unit volume:
ρ = m/V
When an object is placed in a fluid, whether it sinks or floats depends on how its density compares to the fluid's. If the object has a lower density than the fluid, the object floats; 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 for gold is 19.3.
| 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 |
Pressure is force per unit area:
P = F/A
Place a brick on the floor with its largest-area face down. If you then stand it up on its end, it exerts the same force on the floor but a much larger pressure, because the contact area is less.
The force exerted on an object by a fluid is toward the object and perpendicular to its surface. At a microscopic level, the force is associated with the atoms and molecules in the fluid colliding elastically with the object. The atoms and molecules experience a change in momentum in the direction perpendicular to the surface, so there is a net force in that direction. The object applies a force away from itself, and it feels an equal-and-opposite force toward itself.
The SI unit for pressure is the pascal.
1 Pa = 1 N/m2
There are plenty of other pressure units. Converting from one unit to another can be done using the following relationships:
1 atm = 1.01 x 105 Pa = 14.7 lb/in2 = 760 torr
pounds / square inch = lb/in2 is also abbreviated as psi.
1 torr = 1 millimeter of mercury = 1 mm Hg.