Physical quantity is a quantity that can be measured. Physical quantities can be classi ed into two: fundamental quantities and derived quantities.

**Physical
Quantities and Units**

Physical quantity is a quantity that can be
measured. Physical quantities can be classi ed into two: fundamental quantities
and derived quantities. Quantities which cannot be expressed in terms of any
other physical quantities are called fundamental quantities. Example: Length,
mass, time, temperature. Quantities like area, volume and density can be
expressed in terms of some other quantities. They are called derived
quantities.

Physical quantities have a numerical value (a
number) and a unit of measurement (say, 3 kilogram). Suppose you are buying 3
kilograms of vegetable in a shop. Here, 3 is the numerical value and kilogram
is the unit. Let us see about units now.

A unit is the standard quantity with which unknown
quantities are compared. It is defined as a specific magnitude of a physical
quantity that has been adopted by law or convention. For example, feet is the
unit for measuring length. That means, 10 feet is equal to 10 times the
definite predetermined length, called feet. Our forefathers used units like
muzham, furlong (660 feet), mile (5280 feet) to measure length.

Many of the ancient systems of measurement were
based on the dimensions of human body. As a result, unit of measurement varied
from person to person and also from location to location. In earlier time, different
unit systems were used by people from different countries. Some of the unit
systems followed earlier are given below in Table 1.

But, at the end of the Second World War there was a
necessity to use worldwide system of measurement. Hence, SI (International
System of Units) system of units was developed and recommended by General
Conference on Weights and Measures in 1960 for international usage.

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9th Science : Measurement and Measuring Instruments : Physical Quantities and Units |

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