A fluid’s density, often known as its mass density, is defined as the ratio of its mass to its volume.

Density is defined as the mass per unit volume of a fluid.The symbol represents it. The kilogramme per cubic metre is the SI unit for mass density.

The density of liquids is constant, whereas the density of gases varies with changes in pressure and temperature.

Water has a density of 1 gramme per cubic centimetre, or 1000 kilogrammes per cubic metre.


The ratio of a fluid’s weight to its volume is known as its specific weight or weight density. Weight density is defined as the weight per unit volume of a fluid and is symbolised by the sign W.


The volume of a fluid occupied by a unit mass, or volume per unit mass of a fluid, is defined as its specific volume.As a result, the reciprocal of mass density is specific volume.

It’s written as It’s a term that’s widely used to describe gases.


The weight density (or density) of a fluid is defined as the ratio of its weight density (or density) to the weight density (or density) of a standard fluid. Water is used as the standard fluid for liquids, and air is used as the standard fluid for gases. Relative density is another name for specific gravity. It has the sign S and is a dimensionalless quantity.

The density of a liquid is equal to the specific gravity of the fluid multiplied by the density of water if the specific gravity of the fluid is known. Mercury, for example, has a specific gravity of 13.6. As a result, the density of mercury is 13.6 x 1000.


The property of a fluid that provides resistance to the flow of one layer of fluid over another adjacent layer of fluid is known as viscosity. When two layers of a fluid travel at different velocities over each other at a distance apart, the viscosity, together with the relative velocity, causes a shear stress to act across the fluid layers.

The top layer puts pressure on the neighbouring layer, whereas the lower layer puts pressure on the top layer. The rate of change of velocity is related to the shear stress. The symbol represents it.

Where (mu) is the proportionality constant, also known as the dynamic viscosity coefficient or simply viscosity, and The rate of shear strain, shear deformation, or velocity gradient is represented by cre.dynamic viscosity coefficient.

The shear stress required to create a unit rate of shear strain is also known as viscosity.


It is defined as the ratio of a fluid’s dynamic viscosity to its density. The Greek sign is used to represent it.clip was given the value nu.

The unit of kinematic viscosity in MKS and SI is

While it is written as in CGS units, it is written as

Kinematic viscosity is also known as stoke in the CGS system.1 stoke = 1

Newton’s Law of Viscosity:

It states that the rate of change of shear strain is directly proportional to the shear stress on a fluid element layer. The co-efficient of viscosity is the proportionality constant.

Fluids that obey the above relation are called Newtonian fluids, while fluids that do not obey the above relation are referred to as Non-Newtonian fluids.

Variation of Viscosity with temperature:

The viscosity of liquids reduces as the temperature rises, but the viscosity of gases rises as the temperature rises.

Figure: Type of fluids

1. The ideal fluid is: The term “ideal fluid” refers to a fluid that is incompressible and has no viscosity. Because all fluids have some viscosity, the ideal fluid is merely a hypothetical fluid.

2. Genuine Fluids: The term “real fluid” refers to a fluid that has viscosity. In practise, all of the fluids are real fluids.

3. Fluids of Newtonian Mechanics: Shear stress is exactly proportional to rate of shear strain in a real fluid (or velocity gradient).

4. Fluid that is not Newtonian: Shear stress is not related to the rate of shear strain in a real fluid.

5. The Perfect Plastic Fluid: Shear stress is proportional to the rate of shear strain in a fluid with a shear stress greater than the yield value.

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