Physical Properties


Many of the properties of fluids are related to temperature and pressure.  Whether a substance is a solid, liquid or gas depends on these two properties.  The picture shows the triple point of water.

By Mpfiz (Own work) [Public domain], via Wikimedia Commons

There are many properties that affect the behavior of fluids. Some of these properties are related to the weight, size, or stress/rate of deformation characteristics of the fluid. Other properties are related to the temperature and pressure of the fluid. These properties include internal energy, enthalpy, and entropy.


The quantification of how a fluid behaves due to changes in environmental conditions requires precise mathematical definitions of the fluid’s properties.  Some of the properties commonly used with liquids are shown in the figure.  Other properties will be introduced when they are first encountered.



The force exerted on an object due to gravitational attraction.  The weight of an object is different on the moon than on the Earth due to the difference in gravitational attraction caused by the difference in the size of the Earth and moon.


Gravitational Acceleration

The force of attraction between two bodies is known as gravity.  Gravity plays a big role in the behavior of fluids.  In zero gravity, fluids that are not constrained within a container take on the shape of a ball due to surface tension.  On Earth, fluids on a flat surface flatten out because of gravity.  The force of attraction between an object’s mass and the mass of the Earth can be computed using a quantity known as the gravitational acceleration or acceleration due to gravity.  The gravitational acceleration has units of 32.2 ft/sec2 in Imperial units and 9.8 m/sec2 in metric units (see insert).


Mass is a measure of how much of an object exists.  Mass is a constant and doesn’t change when the object is moved from one gravitational or acceleration field to another.  Mass is not measured directly, but is computed based on the weight of an object in a known gravitational field.


Volume is the amount of space occupied by an object.  Volume has the dimensions of length cubed.  The volume occupied by an incompressible fluid does not change when its shape changes.  The volume of a compressible fluid can change due to a change in shape.


Density is the ratio of mass divided by volume.  Changes in density for a liquid are generally very small since volume changes are small.  When the changes in density are negligible compared to other effects, the fluid is said to be incompressible.  Conversely, changes in the density of a gas can be large.  When the density changes cannot be ignored, the fluid is said to be compressible.  At low speeds (Mach numbers), density changes in gasses may be small and the gas can be analyzed as an incompressible fluid.

Specific Weight

The specific weight of an object is similar to the density, with the exception that the weight is divided by the volume.  The ratio of the specific weight to density is the gravitational acceleration, g.  The specific weight is sometimes referred to as the “Unit Weight” (weight per unit volume).

Bulk Modulus

A fluid at rest is subjected only to normal stresses called pressures.  These pressures exist in equilibrium with gravitational and external forces.  The Bulk Modulus is a measure of a fluid’s resistance to a change in hydrostatic pressure (normal stresses are the same in all directions).  The bulk modulus is determined by dividing a change in pressure by the corresponding change in volumetric strain.  The bulk modulus of all fluids is a finite number greater than zero.  However, when a fluid is considered to be incompressible, the bulk modulus takes on a value of infinity.  This is a consequence of zero volume changes associated with incompressible fluids.  The units of the bulk modulus are lb/in2 in Imperial units and N/m2 or Pascals in metric units.


A fluid at rest is subjected only to normal stresses or pressure.  Therefore, fluid statics is concerned only with describing pressure distributions.  However, fluids in motion are subjected to both normal stresses and shearing stresses.  Viscosity is the property that relates the shearing stresses to the rate of deformation (strain rate).  Viscosity can be but does not have to be a constant.

Learning Objectives:

Learn the definitions, mathematical equations and nomenclature used to identify common properties of fluids.