Unit: The chosen standard used for measuring physical quantity is called unit.
Units are of two types : 1. Fundamental Unit and
2. Derived Unit
System of Units : Units depend on choice. Each choice of units leads to a new system (set) of units. The internationally accepted systems are
1. CGS system,
2.MKS System
3. FPS System
4. SI Units.
In SI Units, there are seven fundamental units given in the following table :
| Physical Quantity | SI Unit | Symbol | Physical Quantity | SI Unit | Symbol |
| Length | metre | m | Temperature | kelvin | K |
| Mass | kilogram | kg | Luminuous intensity | candela | Cd |
| Time | second | s | Amount of substance | mole | mol |
| Electric Current | ampere | A |
Besides these seven fundamental units, two supplementary units are also defined, viz ., radian [rad] for plane angle and steradian (sr) for solid 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 units | SI unit | Relation |
| 1 | Force | dyne | newton | 1 newton= 10 5 dyne |
| 2 | Work | erg | joule | 1 joule= 107 erg |
Some practical units of length, mass and time:
| Length | Mass | Time |
| Light year= distance travelled by light in one year in vacuum 1 LY= 9.46 x 1015 m 1 Astronomical Unit (A.U.)= 1.5 x 1011 m 1 Nautical mile or Seamile= 6020 ft 1 Micron= 1 µm= 10-6 m 1 Angstron Å= 10-10 m | 1 Quintol = 102 kg 1 Atomic Mass Unit (amu) or Dalton = 1.66 x 10-27 kg 1 Pound= 0.4537 kg 1 Chandrashekhar limit = 1.4 times the mass of sun= 2.8 x 1030 kg | 1 Solar day = 86400 sec 1 Year= 365 ½ solar days 1 Lunar month=27.3 solar days Tropical year= Year in which total solar eclipse occurs. Leap year= Year in which month of February is of 29 days. |
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