11.1 What Is a Fluid?

A fluid is a state of matter that yields to sideways or shearing forces. Liquids and gases are both fluids. Fluid statics is the physics of stationary fluids.

11.2 Density

Density is the mass per unit volume of a substance or object. In equation form, density is defined as
$ρ = \frac{m}{V} .$
The SI unit of density is ${kg/m}^{3}$ .

Pressure is the force per unit perpendicular area over which the force is applied. In equation form, pressure is defined as
$P = \frac{F}{A} .$
The SI unit of pressure is pascal and $1 Pa = 1 {N/m}^{2}$ .

Pressure is the weight of the fluid $mg$ divided by the area $A$ supporting it (the area of the bottom of the container):
$P = \frac{mg}{A} .$
Pressure due to the weight of a liquid is given by
$P = hρg,$
where $P$ is the pressure, $h$ is the height of the liquid, $ρ$ is the density of the liquid, and $g$ is the acceleration due to gravity.

Pressure is force per unit area.
A change in pressure applied to an enclosed fluid is transmitted undiminished to all portions of the fluid and to the walls of its container.
A hydraulic system is an enclosed fluid system used to exert forces.

Gauge pressure is the pressure relative to atmospheric pressure.
Absolute pressure is the sum of gauge pressure and atmospheric pressure.
Aneroid gauge measures pressure using a bellows-and-spring arrangement connected to the pointer of a calibrated scale.
Open-tube manometers have U-shaped tubes and one end is always open. It is used to measure pressure.
A mercury barometer is a device that measures atmospheric pressure.

Buoyant force is the net upward force on any object in any fluid. If the buoyant force is greater than the object’s weight, the object will rise to the surface and float. If the buoyant force is less than the object’s weight, the object will sink. If the buoyant force equals the object’s weight, the object will remain suspended at that depth. The buoyant force is always present whether the object floats, sinks, or is suspended in a fluid.
Archimedes’ principle states that the buoyant force on an object equals the weight of the fluid it displaces.
Specific gravity is the ratio of the density of an object to a fluid (usually water).

Attractive forces between molecules of the same type are called cohesive forces.
Attractive forces between molecules of different types are called adhesive forces.
Cohesive forces between molecules cause the surface of a liquid to contract to the smallest possible surface area. This general effect is called surface tension.
Capillary action is the tendency of a fluid to be raised or suppressed in a narrow tube, or capillary tube which is due to the relative strength of cohesive and adhesive forces.

Measuring blood pressure is among the most common of all medical examinations.
The pressures in various parts of the body can be measured and often provide valuable medical indicators.
The shape of the eye is maintained by fluid pressure, called intraocular pressure.
When the circulation of fluid in the eye is blocked, it can lead to a buildup in pressure, a condition called glaucoma.
Some of the other pressures in the body are spinal and skull pressures, bladder pressure, pressures in the skeletal system.