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**What is Units and Measurements?**

** Physics is the study of nature and its laws. The word physics has come from the Greek word fusis which means nature. Physics can be explained with the help of following theories.**

✦ **Mechanics**

✦ **Thermodynamics**

✦ **Electromagnetism**

✦ **Theory of relativity**

✦ **Quantum mechanics**

✦ **Electronics**

**Measurement**

**Physical Quantities, Standards and Units**

**Measurement is the fundamental of physics. Measurement is the process of comparison of an unknown quantity with a standard quantity of the same kind. Without measurement there is no law of physics. Laws of physics are expressed in terms of physical quantities such as time, force, temperature, density and numerous others. **

**Physical quantities are often divided into fundamental and derived quantities. Derived quantities are those whose definitions are based on other physical quantities, i.e., fundamental quantities, e.g., speed, area, density, etc. **

**Fundamental quantities are not defined in terms of other physical quantities, e.g., length, mass and time. Physical quantities in general are divided into two classes; (1) Scalar quantities, and (2) Vector quantities.**

** A scalar quantity is one which has only magnitude.**

** A vector quantity has both magnitude and direction.**

__Unit of Length__

**The SI unit of length is the metre (m). Various other metric units used for measuring lengths are related to metre by either multiples or submultiples of 10. Thus,**

**1 kilometre (km) = 1000 (or 10 ^{3})m**

**1 centimetre (cm) = 1/1000 (or 10 ^{-2})m**

**1 millimetre (mm) = 1/1000 (or 10 ^{-3})m**

**Very small distances are measured in micrometres or microns (μm), angstroms (A), nanometers (nm) and femtometres (fm)**

**1μm = 10 ^{-6} m**

**1nm = 10 ^{-9} m**

**1A = 10 ^{-10} m**

**1 fm = 10 ^{-15 }m**

**The light year is a unit of length and is equal to the distance travelled by light in one year.**

**1 light year = 9.46 x 10 ^{15} m**

**= 9.46 x 10 ^{12} km**

**An astronomical unit (A.U) is the mean distance from the centre of the earth to the centre of the sun.**

**1 A.U = 1.495 x 10 ^{11}m**

**A parsec is the distance at which an astronomical unit subtends an angle of one second of arc.**

**1 parsec = 3. 084 x 10 ^{16}m**

__Unit of Mass__

**The SI unit of mass is the kilogram (kg). In practice, multiples and sub – multiples of kilogram are used:**

**1 tonne (t) = 10 ^{3}kg (1000 kg)**

**1 gram (g) = 10 ^{-3} (1/1000 kg)**

**1 milligram (mg) = 10 ^{-6} kg**

__Unit of Time__

**The SI unit of time is the second(s). In the earlier days people used different system of units for measuring length, mass and time.**

**FPS (Foot, Pound and Second) system**

**CGS (Centimetre, Gram and Second) system**

**MKS ( Metre, Kilogram and Second) system**

__British (or FPS) system of units__

**In this system, the unit of length is foot (ft), the unit of mass is pound (lb) and the unit of time is second (s).**

**1 foot = 0.3048 m**

**1 pound = 0.4536 kg**

**The unit of time is same in both the S.I and the F.P.S systems.**

__The CGS System__

**It is another system of measure used in earlier days. In this system unit of length is Centrimetre (cm), Gram (gm) and Second (s).**

__The SI system__

**There are 7 basic quantities in the SI system and 2 supplementary quantities.**

**Quantity Unit**

**Length - Metre (m)**

**Mass - Kilogram (kg)**

**Time - Second (s)**

**Electric Current - Ampere (A)**

**Temperature - Kelvin (K)**

**Luminous Intensity - Candela**

**Amount of substance - Mole (mol)**

**Supplementary quantities**

**Plane Angle – Radian**

**Solid Angle - Steradian**

**Plane angle and solid angle are supplementary quantities till 1995. They were derived quantities now.**

__Derived Quantities__

**Quantities got by the multiplication or division of fundamental physical quantities like length, mass and time are called derived quantities. Apart from these above mentioned derived quantities, there are some other derived quantities like area, volume, density, etc.**

**Important derived units**

**Physical quantity definition SI unit**

**1)Area Length square m ^{2}**

**2)Velocity Displacement – per unit time ms ^{-1}**

**3)force mass X acceleration kgms ^{-2}**

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