
There are three states of matter which are recognised e.g.
solid, liquid, gas. Solid can resist deformation but liquid and gas
can not., so they flow under the action of force. A Fluid is defined as a substance which deforms continuously, or
flows, when subjected to shearing forces. Both liquid and gas come
under the category of fluid. Shear stresses are developed when the particles of fluid move
with respect to one another. In real world shear forces are present
when we consider flow past solid boundaries; aeroplanes, cars, pipe
walls, river channels etc. Some basic definitions are important to understand 1. Specific gravity
or relative density is defined as the ratio of the density of a
substance to the density of some standard substance at a specified
temperature (usually water at 4 °C, and atmospheric pressure having
a density of 1000 kg/m^{3}).
Specific gravity is a dimensionles quantity. 2. Specific weight is the weight of a unit volume of a
substance. Specific weight=specific gravity X acceleration due
to gravity.

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3. Viscosity is a property that represents the internal
resistance of a fluid to motion or the ‘fluidity’. Viscosity is due
to the cohesive forces between the molecules. Viscosity is
measured in two forms; Dynamic (or absolute) and Kinematic. All fluids are viscous in nature. "Newtonian Fluids" obey the linear
relationship given by Newton's law of viscosity. which states that
shear stress is directly proprtional to velocity gradient. 4. Surface
Tension is a property of fluid that is caused by
cohesion of similar molecules. It can also be termed as the
contractive tendency of the surface of a fluid that allows it to
resist an external force. It is because of surface tension that some
objects denser than water can float on the surface of water.
In a fluid mass, each molecule is pulled equally in every direction
by neighboring molecules, resulting in a net force of zero. The
molecules at the surface do not have other molecules on all sides of
them and therefore are pulled inwards. This phenomena develops some
internal pressure and forces the liquid surfaces to contract to the
minimum area.Surface tension has the dimension of force per unit
length, or of energy per unit area.
Surface tension can be defined as the energy required to
stretch a unit change of a surface area.
Surface tension decreases with increase in temperature. 5.
Fluid Pressure is defiend as "the normal force per unit area exerted on an
imaginary or real plane surface in a fluid " Pressure is measured in Pascal (Pa) which is equal to
Newton per sq m. or psi (pound per sq inch)
Do you know
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Continuity equation
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