**Systems of Units
**A system of units is the complete set of units, both fundamental and derived, for all kinds of physical quantities.

We are giving a detailed and clear sheet on all Physics Notes that are very useful to Understand the Basic Physics Concepts.

## Systems of Units – Units and Measurement

### Systems of Units, Measurements, Quantities

The common system of units which is used in mechanics are given below:

(i) **CGS System:
**In this system, the unit of length is centimetre, the unit of mass is gram and the unit of time is second.

(ii) **FPS System:
**In this system, the unit of length is foot, the unit of mass is pound and the unit of time is second.

(iii) **MKS System:
**In this system, the unit of length is metre, the unit of mass is kilogram and the unit of time is second.

(iv) **SI System:
**The system of units, which is accepted Internationally for measurement is the System Internationaled units abbreviated as SI. This system contains seven fundamental units and two supplementary fundamental units.

**Relationship between Some Mechanical SI Unit and Commonly Used Units**

S.No. |
Physical Quantities |
Units |

1. | Length | (a) 1 micrometre = 10^{-6} m
(b) 1 nanometre = 10 (c) 1 angstorm = 10 |

2. | Mass | (a) 1 metric ton = 10^{3} kg
(b) 1 pound = 0.4537 kg (c) 1 amu = 1.66 x 10 |

3. | Volume | (a) 1 litre = 10^{-3} m^{3} |

4. | Force | (a) 1 dyne = 10^{-5 }N
(b) 1 kgf = 9.81 N |

5. | Pressure | (a) 1 kgf-m²= 9.81 Nm^{-2}
(b) 1 mm of Hg = 133 Nm (c) 1 pascal = 1 Nm (d) 1 atmosphere pressure = 76 cm of Hg = 1.01 x 10 |

6. | Work and energy | (a) 1 erg = 10^{-7} J
(b) 1 kgf-m = 9.81 J (c) 1 kWh = 3.6 x 10 (d) 1 eV = 1.6 x 10 |

7. | Power | (a) 1 kgf-ms^{-1} = 9.81 W
(b) 1 horse power = 746 W |

### Some Practical Units of Length

- 1 fermi = 10
^{-15}m - 1 X-ray unit = 10
^{-13}m - 1 astronomical unit = 1.49 x 10
^{11}m (average distance between sun and earth) - 1 light year = 9.46 x 10
^{15}m - 1 parsec = 3.08 x 10
^{16}m = 3.26 light year

**Units and Measurement Topics:
**Measurement requires tools to provide scientists with a quantity. A quantity describes how much of something there is and how many there are.

Physical Quantities and Their Units | Systems of Units |

Dimensions | Significant Figures |

Rounding off | Error |

Combinations of Errors |