Mechanical Properties of fluid refer to the physical behaviour of a fluid, such as force, pressure, density, viscosity, etc. The nature of fluid can be understood when fluid is at rest or in motion. Hence we categorise the study of fluid in two branches – Hydrostatics and Hydrodynamics.

Understanding the Fluid-

Fluid is a substance that can easily flow and have no fixed shape and deform when external force or stress is applied.

Example – Liquid and Gases.

Properties of Fluid-

1. Density- It is defined as Mass per unit Volume. The density of a fluid is non-uniform.
2. Viscosity- It is defined as the amount of resistance provided by the fluid. Viscosity occurs due to friction within the layers of fluids. 
3. Temperature- It determines the degree of Hotness or Coldness of the Body. The device which measures the temperature is a thermometer.
4. Surface Tension- It is the properties of liquid in which it gets minimum surface area. Due to this larger force of attraction between the molecules, the liquid drop occurs as a sphere because its area is minimum.
5. Capillarity- It is the ability of a liquid to flow in a small tube with some cross-section area. Due to this, it can flow in the opposite direction of gravity.
6. Surface Energy- It is the energy possessed by the surface of the liquid.

This energy is calculated at a constant temperature. Surface energy is calculated as a product of surface tension and area.

Surface Energy =S.TxArea.

Types of Fluids-

1. Ideal Fluids – The fluid which cannot be compressed and doesn’t have viscosity.
2. Non-Ideal Fluids- The fluid which came under the category of viscosity.
3. Newtonian Fluids- Those fluids which obey Newton’s law of viscosity.
4. Non-Newtonian Fluids- Those fluids which do not obey Newton’s law of viscosity.
5. Compressible Fluids- Those fluids whose density is variable when the external pressure or force is applied to the fluid.
6. Non-Compressible Fluids- Those fluids whose density is constant when the external pressure or force is applied to the fluid.
7. Ideal Plastic Fluids- The fluid is said to be an ideal plastic fluid when the shear stress is more than the yield value.

Pressure – It is defined as Force per unit area. The area is taken perpendicular to the surface. At the same level, the pressure in the liquid column is the same. 

Pressure depends on the height(h), acceleration due to gravity(g) and Density of liquid(ρ).

So, Pressure = Force; Pressure = ρgh.

Area

The S.I unit of Pressure is Pascal or Newton/m².

Pascal’s Law – This law is given by Blaise Pascal.

This law states that force per unit area at the same level is constant.

This phenomenon is used in Hydraulics brakes, Lifts, etc. and is applicable when fluid is at rest and in a closed container.

Archimedes’s Principle – According to this Principle, when the object is placed in liquid, it experiences the upward force by liquid buoyancy force. This is applicable when the object is fully or partially immersed. The weight of an object in liquid is always less than the object’s actual weight in the air. 

Law of Floatation – When a body is fully or partially placed in a liquid, then the liquid displaced by the body is equal to the weight with which the liquid is placed. There are two forces acting on each other: One is the upthrust Buoyancy(Fb), and the other is in the downward direction the weight (W=Mg).

There are three conditions to be applied for floatation law-

1- Weight is greater than Buoyancy Force; then, the body will sink.

2- Weight is less than Buoyancy Force; then, the body will float.

3- Weight is equal to Buoyancy Force; then, the body will be in equilibrium.

Fluid Flow Equations- It is the rate of flow of fluid per unit area.

This is used to derive the “equation of continuity”.

Equation of continuity-The mass of fluid passing through the unit area unit time is constant. I.e. A1 V1ρ1 = A2V2 ρ2.

If the fluid is incompressible ρ1=ρ2, then the equation is- A1 V1=A2 V2.

Stoke’s Law – According to this law, the backward force acting on the spherical object of radius (r) moving in liquid of viscosity (η) having velocity (V) is given by 

F=6πrηv

Terminal Velocity – It is the maximum constant velocity attained by the object when freely moving in the fluid.  

Torricelli’s Theorem – It is defined as the velocity with which the liquid is coming from any hole or orifice is equal to the force at which the liquid acquires throughout the vertical length.

Velocity = √2gh

g=acceleration due to gravity

h=height of liquid column

Conclusions:

Hence we concluded that the substance which has a tendency to flow is called fluid. There are some examples like liquid and gases. These can flow on the surface of the liquid; this is due to the density of the substance. When a fluid is in motion, then it is called fluid dynamics. Example – water flowing in the tap, a circular ball moving in a liquid column, etc.

There are some properties of fluid – Density, Surface Tension, Viscosity, Temperature, Capillary, etc. These properties tell us about fluid motion and its nature.

There are some types of fluid – Viscous and Non-Viscous fluid, compressible and Incompressible fluid, Ideal and Non-Ideal fluid, and Ideal Plastic fluid.

There are some terminologies, Laws and Principles which define the mechanical properties of the fluid. Some are given below-

Pascal’s Law, Archimedes Principle, Law of Floatation, Stokes Law, Terminal Velocity, Equation of Continuity, Torricelli’s Principle, etc. This phenomenon helps us to understand the different aspects of fluid, and we can apply it in our daily life.