# Systems of Units – Units and Measurement

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

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## 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 = 10-9 m (c)      1 angstorm =  10-10 2. Mass (a)     1 metric ton = 103  kg (b)     1 pound = 0.4537 kg (c)     1 amu = 1.66 x 10-23 3. Volume (a)     1 litre = 10-3 m3 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-2 (c)     1 pascal = 1 Nm-2 (d)     1 atmosphere pressure = 76 cm of Hg = 1.01 x 105 Pascal 6. Work and energy (a)    1 erg = 10-7 J (b)    1 kgf-m = 9.81 J (c)    1 kWh = 3.6 x 106 J (d)    1 eV = 1.6 x 10-19 J 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 1011 m (average distance between sun and earth)
• 1 light year = 9.46 x 1015 m
• 1 parsec = 3.08 x 1016 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.