Scientists and engineers were leaders in proposing SI units, but their adoption caused changes in many areas of life all over the world. Road signs began to be marked in kilometers rather than miles and articles were weighed in kilograms instead of pounds. An agreement that benefited science has also been important in developing international trade and communications. In this system the units are based on specific quantities, which do not vary. For example, in this system:
Unit of Length is defined as the length of the path traveled by light in vacuum during a time interval of1/(2.99792458 X 108) seconds.
Unit of Mass is defined as "the mass of a particular solid cylinder made of platinum-iridium alloy kept in Paris, known as the International Prototype Kilogram".
Unit of time, second, is equal to the duration of 9192631770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium –133 atoms.In all the three definitions given above you will notice that the quantities do not vary and are independent of different environmental factors.
The following table shows the basic units in the SI system together with their symbols:Basic Units in the SI System
| Physical Quantity | Name of the Unit | Symbol |
|---|---|---|
| Length | meter | m |
| Mass | kilogram | kg |
| Time | second | s |
| Temperature | kelvin | K |
| Electric current | ampere | A |
| Luminous intensity | candela | cd |
| Amount of substance | mole | mol |
Usually all small measurements are expressed by using the prefixes - deci, centi, milli, etc. with the units.
For large measurements, we use deca, hecto, kilo etc. as prefixes with the units. The symbol and meaning of each prefix is given below
| Prefix | Symbol | Fraction/Multiple |
|---|---|---|
| Deci | d | 10-1 |
| Centi | c | 10-2 |
| Milli | m | 10-3 |
| Micro | μ | 10-6 |
| Nano | n | 10-9 |
| Pico | p | 10-12 |
| Femto | f | 10-15 |
| Atto | a | 10-18 |
| Deca | da | 101 |
| Hecto | h | 102 |
| Kilo | k | 103 |
| Mega | M | 106 |
| Giga | G | 109 |
| Tera | T | 1012 |
| Peta | P | 1015 |
| Exa | E | 1018 |
It should be noted that while writing the units of a physical quantity certain guide lines have to be followed:
- Use upper case to represent the symbol for a unit named after a scientist.
- Use lowercase to represent a unit not named after a scientist.
- Do not capitalize the first letter of the expansion of a unit named after a scientist.
- Never use plural to represent a unit.
- Insert a period (.) or leave space to represent a compound unit formed by multiplication of two or more units.
- Use negative power or solidus (/) for representing compound units formed by dividing one unit by the other.
- A symbol for unit is represented by the first letter of the unit, except for few units like hertz, Pascal, candela, mole, radian and steradian.
- Never use full stop or comma or colon after the symbol representing the unit.
- Zero should be placed before a decimal number.
- Leave space between a number and a unit.
- A hyphen should be placed in between the number and the unit when the number is used as an adjective.
Certain units of time which are not very commonly used are listed below:
Year
One year is the time which the earth takes for one revolution around the sun which is, equal to 365 days.
Leap Year
A leap year is the year in which the month of February is of 29 days and that year will have 366 days.
Decade
A decade is a period of 10 years.
Century
A century is a period of 100 years.
Millennium
A millennium is a period of 1000 years.
Lunar month
One lunar month is the time in which the moon completes one revolution around the earth.
Units of all other physical quantities can be derived from the basic units and hence are called "derived units". The following table shows the list of various physical quantities, derived formula and corresponding SI Units:Derived Units
| Quantity | Formula | Symbol (SI Unit) |
|---|---|---|
| Area | A=LxB | m2 |
| Volume | V=LxBXH | m3 |
| Density | D = Mass/Volume | kg m- 3 |
| Velocity | V = Distance/Time | m s-1 |
| Acceleration | a = Change in Velocity/Time | m-2 |
| Momentum | p = mass x velocity | Kg ms-1 |
| Force | F = Mass x Arceleration | N(newton) |
| Work | W = Force x Distance | J(joule) |
| Power | P = Work / Time | W (watt) |
| Potential Energy | P.E. = Force x Displacement = m xg xh | J(joule) |
| Kinetic Energy | KE. = (1/2) x mass x (Velocity)2 | J (joule) |
| Moment of Force | Moment = Force x Perpendicular Distance | Nm |
| Pressure | P = Force / Area | N m-2 or Pa(Pascal) |
