## Physical World and Measurements

# SI Units and Measurements of Length, Mass and Time

The set of basic, supplementary and derived units of various physical quantities is called system of units. The most widely used system of units is known as the International system of units.

The common system of units are

**CGS SYSTEM:** In this system, the unit of length is centimetre, 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 metre, the unit of mass is kilogram and the unit of time is second.

**SI SYSTEM:** This system contain 7 fundamental units and 2 supplementary fundamental units. This system is universalization of M. K. S system.

The characteristics of this system are easily reproducible, do not vary with any physical parameters like time , temperature , place and pressure etc. and easily accessible.

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1. Plane angle in radian = \tt \frac{Length\ of\ arc}{radius}

2. Solid angle in steradian = \tt \frac{area\ cut\ out\ on\ the\ surface\ of\ sphere}{(radius)^{2}}

3. Area, A = Length × Length

Physical quantity |
Relation with basic quantities |

1. Area | Length × Breadth |

2. Volume | Length × Breadth × Height |

3. Density | Mass/Volume |

4. Speed or velocity | \tt \nu=\frac{Distance}{time} |

5. Acceleration or Acceleration due to gravity | \tt a=\frac{Velocity}{time} |