Theory:

Nature and natural phenomena are studied in a branch of science called Physics. All science subjects are built on the foundation of physics, which is purely based on experimental observations. These principles and observations give us a better understanding of nature.
 
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Scientific theories are validated based on the results of various experiments. Also, many physical quantities are used in these theories. All scientific researches and experiments begin with measurement.
Measurement is essential in our daily lives.
Measurement is the method of determining an unknown physical quantity (a number) by comparing it to a known quantity (unit).
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A physical quantity is a quantity that measures the properties of material or substances. Some of the physical quantities are length, mass, temperature and volume.
For a perfect measurement, three things are required. They are,
  1. Instrument
  2. Standard quantity
  3. Unit
For instance, if the length of the table is \(1\ metre\), then '\(1\)' is the magnitude and '\(metre\)' is the unit selected to express the length, which is a physical quantity. Here, the meter scale is the instrument used to measure the length. This complete process is known as measurement.
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Unit systems
Measurement is used everywhere around us. Various unit systems are used by people in different parts of the world for measurement. The common systems of units are given below.
 
Unit system
Length
Mass
Time
CGS
centimetre
gram
second
MKS
metre
kilogram
second
FPS
foot
pound
second