Measurement

Physics deals with the study of natural phenomena .To approach the subject quantitatively,it is essential to male measurements.For example,we know that the earth rotates not only about its own axis but also revolves around the sun.What is the period of rotation of the earth about its own axis ?What is the period of revolution of the earth around the sun?To answer these questions,we have to make measurements.

A physical quantity is that which can be measured and expressed in unit directly or indirectly.Mass, length, time, density, temperature, etc are some examples of physical quantities.Sorrow, happiness, sadness, etc cannot be measured .So, they are not physical quantities.Without measurement, we cannot find the exact value of any physical quantity.

For the measurement of any physical quantity, we need a fixed quantity as a standard quantity.The measurement of unknown quantity can be done by comparing it with a standard quantity of the same kind.Thus, the process of comparison of an unknown quantity with a known standard quantity is called measurement.

Importance of measurement

1.Measurement is essential in selling and buying goods and commodities.
2.It is essential in performing scientific experiments to establish truth about a physical phenomenon.
3.It is required for global understanding of the quantity of a substance.
4.It is essential in performing experiment for making our daily foods.
5.Measurement of the medicine is a must in the treatment of diseases.

Fundamental and derived quantity

Those physical quantities, which can neither be derived from others nor be further resolved into other simpler quantities, are called fundamental quantities.
For example,length of a body is a fundamental quantity as it cannot be expressed in-terms of other quantities.How many kilograms equal 1 m?It does not make any sense.Length is not dependent on mass.Length, mass, time, electric current, temperature, intensity of light and amount of substance are the example of fundamental quantities.                                                                                        
Those physical quantities, which depend on two or more fundamental quantitise or power of fundamental quantity are called derived quantities.Area is a derived quantity.Since, area=length * breadth, it depends on two power of length (i.e. l square).
Similarly,density is also a derived quantity as density depends am mass (m) and three power of length (i.e. l cube).Volume, pressure, force, speed, acceleration, etc, are some other examples of derived quantities.