characteristics of Fluid

        CHARACTERISTICS OF FLUID 

FLUID-

                Fluids are divided into liquids and gases. A liquid is hard to compress and

as in the ancient saying ‘Water takes the shape of the vessel containing it’, it

changes its shape according to the shape of its container with an upper free

surface. Gas on the other hand is easy to compress, and fully expands to fill

its container. There is thus no free surface.

Consequently, an important characteristic of a fluid from the viewpoint

of fluid mechanics is its compressibility. Another characteristic is its viscosity.

Whereas a solid shows its elasticity in tension, compression or shearing stress,

a fluid does so only for compression. In other words, a fluid increases its

pressure against compression, trying to retain its original volume. This

characteristic is called compressibility. Furthermore, a fluid shows resistance

whenever two layers slide over each other. This characteristic is called

viscosity.

In general, liquids are called incompressible fluids and gases compressible

fluids. Nevertheless, for liquids, compressibility must be taken into account

whenever they are highly pressurised, and for gases compressibility may be

disregarded whenever the change in pressure is small. Although a fluid is an

aggregate of molecules in constant motion, the mean free path of these

molecules is 0.06pm or so even for air of normal temperature and pressure,

so a fluid is treated as a continuous isotropic substance.

Meanwhile, a non-existent, assumed fluid without either viscosity or compressibility

is called an ideal fluid or perfect fluid. A fluid with compressibility

but without viscosity is occasionally discriminated and called a perfect fluid,

too. Furthermore, a gas subject to Boyle’s-Charles’ law is called a perfect or

ideal gas. 

UNITS AND DIMENSIONS- 

                                                     A11 physical quantities are given by a few fundamental quantities or their combinations. The units of such fundamental quantities are called base 

Units and dimensions 7units, combinations of them being called derived units. The system in which length, mass and time are adopted as the basic quantities, and from which

the units of other quantities are derived, is called the absolute system of

units.  

                           Absolute system of unit system-

                                             MSK system of unit--

                                                                                     This is the system of units where the metre (m) is used for the unit of length, kilogram (kg) for the unit of mass, and second (s) for the unit of time as the base units.

                                             CGS System of unit--

                                                                                  This is the system of units where the centimetre (cm) is used for length, gram (g) for mass, and second (s) for time as the base units.

                                              International standard of unit(SI)-

                                                                                      SI, the abbreviation of La Systkme International d’Unites, is the system developed from the MKS system of units. It is a consistent and reasonable system of units which makes it a rule to adopt only one unit for each of the various quantities used in such fields as science, education and industry. There are seven fundamental SI units, namely: metre (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for thermodynamic temperature, mole (mol) for mass quantity and candela (cd) for intensity of light. Derived units consist of these units.