The comparison of any physical quantity with its standard unit is called **measurement.**

Physics is a quantitative science, based on measurement of physical quantities. Certain physical quantities have been chosen as fundamental or base quantities. The fundamental quantities that are chosen are length, Mass, Time, electric current, thermodynamic temperature, amount of substance, and luminous intensity.

Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen, internationally accepted reference standard called **unit**.

The result of a measurement of a physical quantity is expressed by a number (or numerical measure) accompanied by a unit. Although the number of physical quantities appears to be very large, we need only a limited number of units for expressing all the physical quantities, since they are interrelated with one another. The units for the fundamental or base quantities are called **fundamental **or **base units**.

The units of all other physical quantities can be expressed as combinations of the base units. Such units obtained for the derived quantities are called **derived units**.

A complete set of these units, both the base units and derived units, is known as the **system of units***.*

S.No. | Fundamental Quantities | Fundamental Units | Symbol |
---|---|---|---|

1 | Length | Meter | m |

2 | Mass | kilogram | kg |

3 | Time | second | s |

4 | Temperature | Kelvin | K |

5 | Electric current | ampere | A |

6 | Luminous intensity | candela | cd |

7 | Amount of substance | mole | mol |

## SI Units:

The **International System of Units (SI)** based on seven base units is at present internationally accepted unit system and is widely used throughout the world.

The SI units are used in all physical measurements, for both the base quantities and the derived quantities obtained from them.

SI units used decimal system; conversions within the system are quite simple and convenient.Certain derived units are expressed by means of SI units with special names (such as

Certain derived units are expressed by means of SI units with special names (such as Joule, Newton, watt, etc).

The SI units have well defined and internationally accepted unit symbols (such as m for meter, kg for kilogram, s for second, A for ampere, N for Newton etc).

## The International system of units:

In earlier time scientists of different countries were using different systems of units for measurement. Three such systems, the CGS, the FPS (or British) system and the MKS system were in use extensively till recently.

The base units for length, mass and time in these systems were as follows:

**CGS system**In this system, the unit of length is centimeter, the unit of mass is gram and the unit of time is second.**FPS system**In this system, the unit of length is foot, the unit of mass is pound and the unit of time is second.**MKS system**In this system, the unit of length is meter, the unit of mass is kilogram and the unit of time is second.**SI system**This system contains seven fundamental units and two supplementary units. The SI units are used in all physical measurements, for both the base quantities and the derived quantities obtained from them. Certain derived units are expressed by means of SI units of special names such as Joule, Newton, watt etc.