# Standards of Measurement: Length , Mass and Time.

The laws of physics are expressed as mathematical relationships among physical quantities. Most of these quantities are derived quantities, or they can be expressed as combination of some basic quantities. There are mainly three quantities in physics or mechanics, Length , Mass and Time.

To do calculations and analysis in physics a standard is established for each of these three quantities.
Which are as follows:

Length:
The history of unit of length dates a long back. In A.D. 1120 the king of England decided that the standard of length in his country would be named the yard and would be precisely equal to the distance from the tip of his nose to the end of his outstretched arm. Similarly the unit feet was used in France. Many other systems for measuring length have been developed. As recently as 1960, the length of the meter was defined as the distance between two lines on a specific platinum–iridium bar stored under controlled conditions in France. This standard was abandoned for several reasons, a principal one being that the limited accuracy with which the separation between the lines on the bar can be determined does not meet the current requirements of science and technology. In the 1960s and 1970s, the meter was defined as 1 650 763.73 wavelengths of orange-red light emitted from a krypton-86 lamp. However, in October 1983, the meter (m) was redefined as the distance traveled by light in vacuum during a time of 1/299 792 458 second.

Mass:
The unit of mass Gram was originally derived from the mass of one centimeter cube of water. In 1887 The unit of mass Kilo Gram (KG) is defined as the mass of a specific platinum–iridium alloy cylinder kept at the International Bureau of Weights and Measures at Sevres, France. And since then the definition has not been changed since that time because platinum–iridium is an unusually stable alloy.

Time:
Before 1960, the standard of time was defined in terms of the mean solar day for the
year 1900. (A solar day is the time interval between successive appearances of the Sun
at the highest point it reaches in the sky each day.) The second was defined as $\frac{1}{60}.\frac{1}{60}.\frac{1}{24}$ of a mean solar day. The rotation of the Earth is now known to vary slightly with time, however, and therefore this motion is not a good one to use for defining a time standard. In 1967, the second was redefined to take advantage of the high precision attainable in a device known as an atomic clock, which uses the characteristic frequency of the cesium-133 atom as the “reference clock.” The second (s) is now defined as 9 192 631 770 times the period of vibration of radiation from the cesium atom.

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