Fluid Pressure


Anything that can flow is called fluid.


  • Molecules in a fluid are arranged in random order.
  • Fluid can’t withstand tangential or shearing stress an indefinite period. It began to flow when shearing stress is applied.
  • It has no definite shape of its own.
  • A force can assert a force in a direction perpendicular to its surface.

Difference between Liquid and Gas



Liquid is incompressible and has a definite volume. Gas is compressible and expand to occupy space.
Diffusion rate is very slow. Diffusion rate is very high.
Have low kinetic energy. Have high kinetic energy.


The total force asserted by a liquid on any surface in contact with it is called thrust.

It is a vector quantity.

SI Unit – N

CGS Unit – Dyne

Dimension –  [MLT^-2]


Pressure is a ratio of force and area.

It is a scalar quantity.

SI Unit – Pa

CGS Unit – dyne/cm^2

Dimension – [ML^-1T^2]

  • A sharp knife cuts better than a blunt one.
  • The railway tracks are laid on wooden sleepers.
  • It is difficult a man to walk on the sand while a camel walks easily on sand.
  • Pins and nails are made to have pointed ends.


It is a ratio of mass and volume.

It is a scaler quantity.

SI Unit = kg/m^3

CGS Unit = g/cm^3

Dimension = [M^1L^3T^-0]

Relative Density

Relative Density = Density of Substance/Density of Water at 4°C


It is the property of the liquid by which an internal force of friction comes into play when liquid is in motion which opposes the relative motion between different layers.

Coefficient of Viscosity

  • The rate of change of velocity with respect to distance is called the velocity gradient.
  • The viscous force act tangentially between two layers in the opposite direction of velocity.
  • It is found that F is directly proportional to A.

It is a scalar quantity.

SI Unit = kg m^-1 s^-1

CGS Unit = poise

Variation of Viscosity with Temperature and Pressure

Effect of Temperature on Viscosity


  • When the liquid is heated kinetic energy of molecules increases but the intermolecular force between them decreases.
  • gases the viscosity varies due to the diffusion of molecules but the rate of diffusion of a gas is directly proportional to √T.


Effect of Pressure on Viscosity

  • Gases are independent of the pressure
  • Except for water viscosity of liquid increases with an increase in the pressure.
  • In the case of water, viscosity decreases with an increase in pressure.

Stook’s Law

According to stook’s law backward, the viscous force acting on a small spherical ball of radius r moving with uniform velocity v through the fluid of viscosity η.

         Fv = 6πηrv

Terminal Velocity

The constant velocity acquired by a body while falling through a viscous medium is called terminal velocity.

Streamline Flow

When a liquid flows such that each particle of the liquid passing a given point moves along the same path and has the same velocity as it’s predecessor. The flow is called a streamline flow.

Characteristics of Streamline Flow

  • This type of flow is more viscous than turbulent flow.
  • The velocity profile is parabolic in shape.
  • Fluid particles do not cross the path of each other.
  • Fluid particles flow along streamline.

Terminal Flow

When a liquid velocity exceeds a certain limiting value then liquid flow becomes zig-zag. The path and the velocity of a liquid change continuously this flow is called turbulent flow.

Critical Flow

The certain velocity of the liquid if a limiting value of its velocity of the flow is up to which flow is streamline and above which the flow becomes turbulent.

  • It depends on critical velocity.
  • Coefficient of the viscosity of the liquid.
  • The density of the liquid.
  • The diameter of the tube.

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