We come across many things in our daily life that we need to measure. To go to the market in a rickshaw or a taxi we must know its approximate distance from our house.
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Only then would we have an idea of the expected fare. Similarly, to buy a cloth for our clothes, we should know the approximate length of cloth required. In certain cases we need to know exact values. For example, when we have to buy a pair of shoes, we require the exact size, or when we go to the tailor also requires the exact measurements. Any quantity that we study in physics is known as a physical quantity. Measurement of a physical quantity means comparing it with a known standard quantity. Each accepted standard quantity is a standard of measurement. It is called a unit.The measurement of a particular quantity is the same at all places and at all times. For this purpose, scientists have introduced some standard units for making physical measurements. The standard unit of a physical quantity should have the following characteristics.Fistly it is of a convinient size and secondly Its value dies not change with respect to place or time.
A measurement consists of two parts-the magnitude and the units. For example, the length of a cloth is 5 meters. It means that 5 is the magnitude and meter is the standard unit. The study of physics involves measurement of various types. This is why physics is called the science of measurement. To make measurement more scientific, convenient and uniform, in 1791 the French Academy of Science devised the metric system based on the decimal system. In October 1990, the 11th General Conference of Weight and Measures adopted an international system of units known as the SI system of units.