The branch of science which deals with the study of natural phenomena is called Physics.
From the middle of the $18^{th}$ century, natural science has made a rapid progress. So, it has been necessary to divide into different sub-branches. Nowadays Physics means only the branch of Natural Science which deals with the properties of matter, energy and relationship between them.
Physics is very huge, interesting, exciting and complex subject. The range of the mass, length and time is very large in physics. For Example:
In Astronomy : Mass (M) = $10^{30}\;kg$; Length (L) = $10^{26}\;m$; Time (T) = $10^{9}\;years$
In Sub-Atomic: Mass (M) = $10^{-31}\;kg$; Length (L) = $10^{-5}\;m$; Time (T) = $10^{-20}\;years$
To understand physics we must be able to connect our theoretical description of nature with our experimental observations of nature. This connection is made through quantitative measurements. To know the physical quantity quantitatively, measurement is must. In order to measure & test physical quantities we need to define some standard measures. Without a widely accepted system of units of measurement our civilization could not exist.
The comparison of any physical quantity (units) with its standard unit (known fixed quantity) is called measurement.
A physical quantity is a property of an object that can be measured with a measuring instrument. Length, time, mass, velocity, force etc. are the example of physical quantities (units).
The quantity used as a standard of measurement is called the unit.
Physical quantities can be divided into two categories, are as follows:
1) Base (Fundamental) Quantities:
Basic quantities are the fundamental quantities that are not related to each other and that are used to derive all other quantities. There are seven basic quantities are as follows: Mass, Time, Length, Temperature, electric current, intensity of light & quantity of matter.
2) Derived Quantities:
Derived quantities are just quantities that are derived from one or more basic quantities. For example:
Area is a derived quantity because it is derived from the basic quantity length.
i.e. Area = Length * Length.
Force, Velocity, Pressure, Volume etc. are the derived quantities.
This is a recent upgrade about the Standard Units. Click here.
» Dimension of Physical Quantities; » Main uses and its Limitations.
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