properties of fluids in fluid mechanics

Properties of fluids with definitions, formula, and SI units

Fluid mechanics is the branch of science which deals with the study of behaviour and properties of fluids. The fluid may be liquid or gas. Thus fluid mechanics deals with the static, kinematics and dynamic behavior of fluids.

The study of fluids at-rest position is called as fluids statics and in motion called as fluid kinematics. In motion pressure forces are not considered for pressure kinematics. If we consider the forces also in motion then this will come under fluid dynamics.

Properties of fluids

There are many properties of fluids that defines the properties of fluids. We will discuss all of them one by one…

Density or mass density

Density has another name as mass density. The Density of fluid can be defined as the ratio of the mass of the fluid to the volume of the fluid.

Or also can be defined as mass per unit volume of the fluid.

Density ( mass density ) is denoted by ρ (rho).

Unit of Density

In the SI system, the unit of density (mass density) is kilogram per cubic meter. It will be written as kg/m³.

Note- The density of liquid may be considered as constant but for gases, it can be varied with that of pressure and temperature.

Formula of density (Mass density) is… ρ=mass of fluid/volume of fluid

Specific weight or weight density

Specific weight can also called as weight density. Specific weight is the ratio of the weight of the fluid to the volume of the fluid.

Or it can also say as the weight per unit volume of a fluid is called the weight density.

Specific weight is denoted by the symbol w.

Formula for specific density

w= Weight of fluid/Volume of fluid

or =( Mass of fluid)×Acceleration due to gravity/Volume of fluid

or=Mass of fluid×g/Volume of fluid



Formula for specific density = w=ρ×g

Specific volume

Specific volume of a fluid is the volume of a fluid occupied by a unit mass or volume per unit mass of a fluid is called specific volume.

Formula for specific volume  V =1/ρ


Specific volume = Volume of fluid/Mass of fluid

                               =1/(Massof fluid/volume of fluid)


Specific Gravity

Specific gravity is simply the ratio of the density (or weight density) of a liquid to the density (or weight density) of a standard fluid. (Note – This standard fluid can be liquid or gas depends on the condition).

Specific gravity is denoted by the letter S.

Formula for specific gravity is – S=Weight density of liquid (gas)/Weight density of water (air)

Questions based on the properties of the fluids

Question: 01 Calculate the specific weight, density and specific gravity of one litre of a liquid that weighs 7N.


Volume = 1 Litre or 1000 m³

Weight= 7N

1) Specific Weight (w)= Weight / Volume

= 7N/(1/1000)m³

= 7000N/m³

2)Density= w/g

= 7000/9.81 kg/m³

= 713.5 kg/m³

3)Specific gravity= Density of liquid/ Density of water

= 713.5/1000

= 0.7135


Properties of fluids depending upon weight density or density, specific weight, specific volume.

Read more topics of fluid.

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