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1. Units and Measurements

The nature around us is colorful and diverse. It contains phenomena of large varieties as the wind, the sand, the water, the planets, the rainbow, the function of human body, the energy coming from the Sun and the nucleus. 
Physics is the study of nature and its laws. It is an experimental science. We expect that all these different events in nature take place according to some basic laws. Physics is concerned with the basic rules which are applicable to all domains of life.
The nature around us is like a big chess game played by nature. The events in the nature all like the moves of the great game. We are allowed to watch the events of nature and guess at the basic rules according to which the events take place. We may come across new events which do not follow the rules guessed earlier and we may have to declare the old rules in applicable or wrong and discover new rules.
Since physics is the measurement of any mate matters of this whole universe, it is real. No one has been given the authority to frame the rules of Physics. We only discover the rules that are operating in nature. 
Mathematics is the language of Physics. Without knowledge of mathematics it would be much more difficult to discover, understand and explain the law of nature. However, mathematics itself is not Physics. We use a language to express or ideas.
Physics is not a collection of facts and principles; it is the process by which we arrive at general principles that describe how the physical universe behaves. No theory is ever regarded in the final or ultimate truth.
The possibility always exists that new observations will require that a theory be revised or discarded. It is in the nature of physical theory that we can disprove a theory by finding behavior that is inconsistent with it, but we can never prove that a theory is always correct. Every physical theory has a range of validity, outside of which is not applicable.

Units and Dimensions
Physical Quantities
All those quantities which are used to express different natural events and laws of Physics are known as physical quantities.
The quantities such as mass, length, time, movement, energy etc. are called physical quantities. The laws of Physics can be expressed by measuring such quantities.
A physical quantity should be known both qualitatively and quantitatively. 
Lord Kelvin highlighted the importance of quantitative knowledge of a physical quantity. He remarked it as "When you can measure what you are speaking about and express it in numbers, you know something about it, but when you can not express it in number, your knowledge is of unsatisfactory kind."

Physical quantities can be divided into two groups. They are commonly known as;

1. Fundamental Quantities
The physical quantities which are independent and can't be expressed with other quantities are known as fundamental quantities such as length, mass and time.
The unit of mass, length and time are known as fundamental units. These are insufficient to express the units of all the physical quantities, then, adopted few more basic units. All together, there are seven fundamental quantities and two supplementary units. They are given below.

Name of the unit
Electric Current
Amount of substance
Luminous intensity

The Two supplementary units are;
Plane angle
Solid angle

2. Derived Quantities
All other physical quantities which are expressed with the help of fundamental quantities are known as derived physical quantities and their units are known as derived units. For Example;

Some Definition of Basic Units
i. Meter
It is unit of length. The distance traveled by light in vacuum in 1/299,792,458 of a second is called 1m.

ii. Kilogram 
The mass of a cylinder made of platinum-iridium alloy kept at International Bureau of Weights and Measures is defined as 1kg.

iii. Second
The fraction 1/86,400 of the time interval between the consecutive passages of sun though zenith.

iv. Kelvin 
The fraction 1/273.16 of the thermodynamic temperature of triple point of water is called 1K.

v. Mole
The amount of a substance that contains as many elementary entities (molecules or atoms) as there  are number of atoms in 0.012kg of carbon -12 is called one mole. The number is called Avogadro's constant and the best value available is 6.02×1023.

vi. Candela
Luminous intensity of 1 cd is the luminous intensity of a blackbody of surface area 1/600,000 m² placed at the temperature of freezing platinum and at pressure of 101,325N/m², in the direction perpendicular to its surface.