Units of measurement
A unit is well defined magnitude in physics for quantifying the properties of matter. These units are used for measurement of physical quantities, provided that these measures does not undergo a change in the course of time. An important point to remember about the units is that these are acceptable as standards through the scientific community for measurement process and are convenient for use. Standard Units of measurement have fixed values and cannot be changed.
The standard units for three main fundamental quantities in physics that are mostly used to measure other derived quantities are, mass, length and time. So to make a start let’s define these quantities in terms of SI as Kilogram, meter and second.
Kilogram (for mass): It is the mass of a hunk of Platinum-Iridium alloy that has been housed at the International Bureau of Weights and Measures in Sevres, France since 1889.
The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.
Reference: https://home.cern/news/news/cern/how-much-does-kilogram-weigh
Meter is the SI unit of length. It is defined in terms of the numerical value of velocity of light in vacuum 299 792 458 expressed in the unit m/s, where the second is defined in terms of the cesium frequency.
Second is the SI unit of time. It is defined in terms of fixed numerical value of the cesium frequency , the unperturbed ground state hyperfine transition frequency of the cesium-133 atom, to be 91 92 631 770 when expressed in the unit Hz (equal to 1s).
The units for four other fundamental quantities i.e., electric current, amount of substance, thermodynamic temperature, and luminous intensity are Ampere, Mole, Kelvin, and Candela
Download SI Brochure for The International System Units from here:
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