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.
A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same quantity. Any other value of that quantity can be expressed as a simple multiple of the unit of measurement. For example, length is a physical quantity.
Four different systems of units are commonly used in science and engineering. The difference among them arises from the difference in the units used to represent the three fundamental quantities, namely length (l), mass (m) and time (t).
The base units for length, mass and time in these systems were as follows :
• In CGS system they were centimetre, gram and second respectively.
• In FPS system they were foot, pound and second respectively.
• In MKS system they were metre, kilogram and second respectively
SI Units
SI Base Units
| Base quantity |
Name |
Symbol |
| length |
meter |
m |
| mass |
kilogram |
kg |
| time |
second |
s |
| electric current |
ampere |
A |
| thermodynamic temperature |
kelvin |
K |
| amount of substance |
mole |
mol |
| luminous intensity |
candela |
cd |
|
SI Derived Units:-
| area |
square meter |
m2 |
| volume |
cubic meter |
m3 |
| speed, velocity |
meter per second |
m/s |
| acceleration |
meter per second squared |
m/s2 |
| wave number |
reciprocal meter |
m-1 |
| mass density |
kilogram per cubic meter |
kg/m3 |
| specific volume |
cubic meter per kilogram |
m3/kg |
| current density |
ampere per square meter |
A/m2 |
| magnetic field strength |
ampere per meter |
A/m |
| amount-of-substance concentration |
mole per cubic meter |
mol/m3 |
| luminance |
candela per square meter |
cd/m2 |
MKS System
MKS unit of measurement is meter-kilogram-second. SI unit was derived from MKS system. In MKS system length is in meter (m), area is in square meter (m2 ), volume is in cubic meter (m3 ), time in second (s), mass is in kilogram (kg), weight (force) is in kilogram-meter per second square (kg-m/s2 ), density is in kilogram per centimeter square (kg/cm2 ), acceleration is in kilogram-meter per second square (kgm/s2 ), force(pressure) is in kilogram-force per square centimeter (kgf/cm2 ).
CGS System
The CGS system was introduced formally by the British Association for the Advancement of Science in 1874. It found almost immediate favor with working scientists, and it was the system most commonly used in scientific work for many years. Meanwhile, the further development of the metric system was based on meter and kilogram standards created and distributed in 1889 by the International Bureau of Weights and Measures (BIPM). During the 20th century, metric units based on the meter and kilogram–the MKS units–were used more and more in commercial transactions, engineering, and other practical areas.
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