Unit 6. Measurements in physics

Physics is a subject which deals with electricity, magnetism, heat, light, sound, mechanics and properties of matter. In all these branches of physics very little progress was made until measurements were attempted, and today it is generally recognized that a trained physicist must be able to measure accurately distances, volumes, masses, time and temperature as well as other quantities such as electric current and voltage. Every physical quantity has its own unit of measurement. Without a unit accuracy is impossible. To measure the size of anything means to compare it with the standard with witch other people compare the size of things measured by them. For instance, we measure the length of a table by comparing it with the length of a metre.

The way in which we choose to define a unit is left to our own convenience. In England the fundamental unit of length is the yard. In Russia, on the other hand, the standard of length is the meter. The system of measurement based on the metre was designed for simplicity and other units used are obtained from the metre by multiplying or dividing by tens. It is a decimal system. So, in the course of time, different countries have chosen their own values for the fundamental units. In the 20^th century scientists agreed to use a particular system in science, called the metre-kilogramme-second, or M.K.S. system.

The International System of Units (SI) is generally a system of units of measurement devised around seven base units and the convenience of the number ten. It was established in 1960, based on the metre-kilogramme-second system. The International System of Units is declared as an evolving system. Prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses, and as the precision of measurements improves.

SI is used both in everyday commerce and in scientific work, wherever it is carried out. It has been nearly globally adopted. The United Kingdom has officially partially adopted metrication, with no intention of replacing imperial units entirely.

The International System of Units consists of a set of units together with a set of prefixes. The units are divided into two classes – base units and derived units. There are seven base units, each representing, by convention, different kinds of physical quantities: metre (length), kilogram (mass), second (time), ampere (electric current), Kelvin (thermodynamic temperature), candela (luminous intensity), mole (amount of substance).

MKS Units

Length	1 metre (m)	1 centimetre (cm)	1 kilometre (km)
Area	1 square metre (m²)	cm²	km²
Volume	1 cubic metre (m³)	cm³	-
Velocity	1 metre per second (m/sec)	-	1 kilometre per hour (km/h)
Mass	1 kilogram (kg)	1 gram (gr)	1 ton
Density	1 kilogram per cubic metre (kg/ m³)	gr/ cm³	-