UNITS

To measure a physical quantity and express its value in numbers, we need to compare the quantity with a standard called unit. When we say that the length of a rod is 2 meters, it means the length of rod is twice the length of a standard which we have accepted as one meters.

It turns out that units of only seven different physical are required to express the unit of all other physical quantities. Units of these seven quantities are called” base units”

Units of all other quantities are called “derived quantities”

Units of length, mass, time together with units of temperature, electric current, luminous intensity and amount of substance are regarded as base units. Units of length, mass and time are regarded as base units. Units of length, mass, and time are regarded as fundamental.

Example of derived units: units of velocity, acceleration, force, etc. are derived unit. They are derived from and contain the units of length, time and mass.

Characteristics of a standard units

Since trade, industry and markets have become global, there are some common features a standard unit must posses. These are as follows

1.It should be well defined and of a suitable size.

2.It should be easily reproducible in laboratories and must not depend on place and time.

3.It should be acceptable and agreed upon internationally so that results of measurement at different places can be compared.