Mechanical Properties of Fluids: What is Hydrostatic, What is Pascal՚S Law (For CBSE, ICSE, IAS, NET, NRA 2022)

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  • Fluids are the substances which move or flow in a certain direction when an external force is applied to them. Fluids can be either gases or liquids.
  • Fluid is the name given to a substance which begins to flow when external force is applied on it.
  • However, when we study fluids and their mechanical properties, we come across two topics namely hydrodynamics and hydrostatics.
  • Fluids possess characteristic physical properties that govern how they behave when forces are applied to them.
  • Of particular interest are properties that govern fluid responses to ordinary mechanical forces; So, we will not consider electrical and magnetic phenomena.

Mechanical Properties of Fluids

  • Fluids are divided into two classes distinguished by the names of liquids, and elastic fluids or gases, which later comprehends the air of the atmosphere and all the various kinds of air with which chemistry makes us acquainted.
  • Fluids show the effects of gravitation in a more perfect manner than solid bodies; the strong cohesive attraction of the particles of the latter in some measure counteracting the effect of gravity.

What is Hydrostatic?

  • Hydrostatic is basically concerned with fluids at rest while hydrodynamics deals with fluids which are in motion.
  • Fluid statics or hydrostatics is the branch of fluid mechanics that studies “fluids at rest and the pressure in a fluid or exerted by a fluid on an immersed body”

What is Pascal՚S Law?

  • When a fluid is at rest, the pressure on the liquid is the same at all points provided, they are of the same height.
  • Pascal՚s law is a principle in fluid mechanics given by Blaise Pascal that states that a pressure change at any point in a confined incompressible fluid is transmitted throughout the fluid such that the same change occurs everywhere.
  • The change in pressure which has been applied on the fluid which is enclosed can be transmitted undiminished to every point of the fluid and the container vessel՚s walls.
  • When an incompressible fluid is passing between every second in a pipe of non-uniform cross-section, the volume will be the same as the steady flow.
  • Bernoulli՚s principle is an equation which states the conservation of energy of non-viscous fluid motion in a steady state.


  • P = Pressure
  • F = force
  • A = area

What is Reynold՚S Number?

The Reynold՚S Number
  • Reynolds number is referred to the onset of turbulence in a fluid flow which can be determined by a dimensionless parameter.
  • The Reynolds number helps predict flow patterns in different fluid flow situations. At low Reynolds numbers, flows tend to be dominated by laminar flow, while at high Reynolds numbers flows tend to be turbulent.

What is Surface Tension?

  • It is referred to the force per unit length which is acting on the plane of the interface between the bonding surface and the liquid.
  • Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible.
  • It is the amount of the extra energy which the molecules at the interface have when compared to the interior.
  • Surface tension allows insects, to float and slide on a water surface without becoming even partly submerged.