Understanding Units in Physics

 

Courtesy- Freepik

The Importance of Units:

Units, a standardized system for expressing measurements, ensuring uniformity and clarity in communication. They quantify physical quantities such as length, time, mass, velocity, acceleration, energy, and many others.

Unit should be:

1. Well defined

2. Easy to reproduce

3. Easy to compare

4. Internationally accepted

5. Independent of changes in physical conditions

Units are of two types:

1. Fundamental Unit

2. Derived Unit

The International System of Units:

Units depend on choice. Each choice of units leads to a new system (set) of units. The internationally accepted system are 1. CGS(Centimeter Gram Second) system, 2. MKS (Meter Kilogram Second) System, 3. FPS(foot-pound-second) System 4. SI(Système international) Units.

In SI(Système international) Units, there are 7 fundamental units given below:

S.No.
Physical Quantity
SI Unit
Symbol
1.
Length
metre
m
2.
Mass
Kilogram
Kg
3.
Time
Second
S
4.
Electric Current
Ampere
A
5.
Temperature
Kelvin
K
6.
Luminuous intensity
Candela
Cd
7.
Amount of substance
Mole
mol

Beside these seven fundamental units, two supplementary units are also defined e.g. radian(rad) for plane angle and steradian(sr) for sodil angle.
-All the units which are defined/expressed in terms of fundamental units are called derived units.
some Important derived units.

S.No.	Physical Quantity	Cgs Unit	SI Unit	Relation
1	Force	Dyne	Newton	1 newton=105 dyne
2	Work	Erg	Joule	1 joule = 107 erg

 
Some Practical units of length, mass and time
 
 
 
	 
S.No.
 
	 
Length
 
	 
Mass
 
	 
Time
 
 
 
	 
1.
 
	 
Light Year- Distance travel by light in one year in vaccum. 1LY=9.46*105m
 
 
1 Quintol=102kg
 
  
1 Solar day = 86400 sec.
 
 

	

 
	 
2.
 
	 
1 Astronomical nit(A.U.)=1.5*1011m
 
	 
1 Metric Ton = 103kg
 
	 
1 Year = 365 ½ solar days
 
 
 
	 
3.
 
	 
1 Parsec = 3.26 ly=3.08*1016m
 
	 
1 Atomic Mass Unit(amu) or Dalton = 1.66*10-27kg
 
	 
1 Lunar month = 27.3 solar Days.
 
 
 
	 
4.
 
	 
1 Nautical mile or Seamile = 6020 ft.
 
	 
1 Slug=14.59kg
 
	 
Tropical year = It is the year in which total solar eclipse occurs.