Measurement

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Physics Newtonian Mechanics Measurement

	Physical Quantities, Standards, and Units:
	A physical quantity is the property of a substance that can be quantified in physical means. It is always expressed in terms of a numerical value associated with a unit. A quantity used as a standard of measurement is called unit. The standard of measurement is something which is well defined for some particular physical quantities which are necessary. For example mass, length and time. The International System of Units: In 1960, an international committee established a set of standards for length, mass and other basic quantities. The system established is an adaptation of the metric system, and it is called the SI system of units. In this system the units of length, mass and time are the meter, kilogram, and second respectively. Other SI standards established by the committee are those for temperature (Kelvin), electric current (ampere), luminous intensity (candela), and the amount of substance (mole). The Standard of Time: The basic SI unit of time is second, which is defined as 9192631770 times the period of vibration of radiation from the cesium-133 atom. This is the reference clock for time and was defined in the year 1967. Here, period is defined as the time interval needed for one complete vibration. The Standard of Length: The basic SI unit of length is meter. One meter is defined as the distance traveled by light in vacuum during a time of 1/299792458 second. This definition of meter was established in October 1983. As per this definition, the speed of light in vacuum is 299792458 m per second. Before October 1983, the standard of length one meter was regarded as 1650763.73 times the wavelength of orange red light emitted from Krypton-86 lamp. This definition of meter was in effect between 1960s and 1970s. The Standard of Mass: The basic SI unit of mass is one kilogram. One kilogram is defined as the mass of a specific platinum-iridium alloy cylinder kept at the International Bureau of Weights and Measures at Sevres, France. This mass standard was established in 1987. Since then it has not been changed as the alloy is unusually stable. And a duplicate of this is kept at National Institute of Standards and Technology, Maryland.