What is a fluid?

A fluid is anything that can flow. Liquids are certainly fluids, but so are gases.

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
Iron7900
Mercury (the metal)13600
Earth (the planet, on average)5500
Sun (on average) 1400
Black hole 1019

Pressure

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 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.