# measurement

1 Units and Measurements Physics is the study of all aspects of the universe. Physics is an experimental science as careful and accurate measurement is usually required in Physics. Length, mass, and time play a fundamental role in describing nature, and these are the three quantities on which we base our measurements. Length, mass, and time are called dimensions. A unit has to be defined before any kind of measurement can be made. The system of unit which has now gained universal acceptance is the Systeme International d’Unites, usually called SI units or ’The metric system’. The definitions of the basic units of the metric system have evolved over the years. Some of the definitions were cumbersome and hard to duplicate precisely, and by international agreement they have been replaced with more refined definitions. In this system, the unit of length is the meter (m), the unit of time is the second (s), and the unit of mass is the kilogram (kg). Units for other quantities, such as force, energy, and power, are derived from these basic units. Mass is a measure of how much ”stuff we have. It is proportional to the weight of an object, but weight is a force and has a different meaning from mass. Physical quantities are often divided into fundamental quantities and derived quantities. Correspondingly, there are fundamental and derived units. The International System of units is based on seven fundamental units; these are given in Table 1. Table 1: Fundamental quantities and their units Quantity Name of unit Abbreviation Length metre m Mass kilogram kg Time second s Temperature kelvin K Current ampere A Amount of substance mole mol Luminous intensity candela cd Derived units are products or quotients of fundamental units. Some