Introduction:

Measurement of any physical quantity involves comparison with a certain basic, arbitrary chosen, Internationally accepted reference standard called units. Any measured quantity is expressed by a number and a unit. The units for the fundamental or base quantities are called fundamental or base units. The units of all other physical quantities can be expressed as combinations of base units are called derived units. A complete set of these units, both the base units and derived units, is known as the system of units.

Need for measurement

There is a need for measurement in physical quantity. Without it, how can you define things? By measuring a physical quantity we define the properties of a material which the physical quantity is associated with. For example, distance, speed, mass, pressure, force, momentum, and energy.

Think this yourself, what is going to be the meaning of the length of an object if we cannot measure it. We will then inform its length subjectively as ‘long’, ‘short', etc. That makes no sense anymore now.

A physical quantity is a property that can be quantified by the measurement of a material or system. A physical quantity may be represented as the combination of a unit and a numerical value. For example, by specifying methods for calculating them, we outline distance and time, while we describe average speed by stating that it is measured as traveled distance divided by travel time. Measures of physical quantities are represented in unit units, which are numerical meanings

Physical quantities haven't just existed before the dawn of mankind. We made them up to serve our own inquisitive purposes. We have discovered that there are certain properties in the world around us, which can be altered in proportions but not in nature.

Heavier. More. Higher. Stronger. These are all comparative terms. And to compare, we need a basis for measurements to take place. Because, without one, there is no logical way to distinguish between different levels or amounts of the same property.

Units of measurement

 There are two types of units of measurement.

  • Fundamental/ Base units
  • Derived units

Fundamental units: The fundamental units are the base units defined by the International System of Units. These units are not derived from any other unit, therefore they are called fundamental units. There are seven fundamental units.

Derived units. The quantities that are derived using the fundamental quantities are called derived quantities. The units that are used to measure these derived quantities are called derived units. Examples: Force, Velocity, Density, Heat, Power, Energy, Momentum, Acceleration. Some derived units are given in the table below.

Difference between fundamental and derived units

System of units:

In earlier times scientists of different countries were using different systems of units for measurement.  We have here four types of system of units, one of which is S.I units which is accepted globally.

System of units are classified mainly into four types:

 1. C.G.S. system: It stands for Centimetre-Gram-Second system. In this system, length, mass and time are measured in centimeters, grams and seconds respectively.

2. M.K.S. system: It stands for Metre-Kilogram-Second system. In this system, length, mass and time are measured in meters, kilograms and seconds respectively.

 3. F.P.S. system: It stands for Foot-Pound-Second system. In this system, length, mass and time are measured in the foot, pound and second respectively.

 4. S.I. system: It stands for System International. This system has replaced all other systems mentioned above. It has been internationally accepted and is being used all over the world. As the SI units use a decimal system, conversion within the system is very simple and convenient.