﻿ A great physicist of this century (P.A.M. Dirac) loved playing with numerical values ofFundamental constants of nature. This led him to an interesting observation. Dirac found that from the basic constants of atomic physics (c, e, mass of electron, mass of proton) and the gravitational constant G, he could arrive at a number with the dimension of time. Further, it was a very large number, its magnitude being close to the present estimate on the age of the universe (~15 billion years). From the table of fundamental constants in this book, try to see if you too can construct this number(or any other interesting number you can think of ). If its coincidence with the age ofthe universe were significant, what would this imply for the constancy of fundamentalconstant.  from Physics Units and Measurement Class 11 Manipur Board

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Units and Measurement

Physics Part I

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A great physicist of this century (P.A.M. Dirac) loved playing with numerical values of
Fundamental constants of nature. This led him to an interesting observation. Dirac found that from the basic constants of atomic physics (c, e, mass of electron, mass of proton) and the gravitational constant G, he could arrive at a number with the dimension of time. Further, it was a very large number, its magnitude being close to the present estimate on the age of the universe (~15 billion years). From the table of fundamental constants in this book, try to see if you too can construct this number(or any other interesting number you can think of ). If its coincidence with the age of
the universe were significant, what would this imply for the constancy of fundamental
constant.

Putting these values in the above equation, we get

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Define physical quantity.

A measurable quantity in terms of which laws of Physics can be expressed is called physical quantity.

The result of a measurement of a physical quantity is expressed by a number accompanied by a unit.

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Give four examples of physical quantities.

Examples of Physical quantitie are force, mass, density and power.
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What is measurement?

Measurement is the process of attaching a numeric value to an aspect of a natural phenomenon. Measurement is making the quantitative knowledge of a physical quantity.
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Define unit.

The quantity used as a standard of measurement is called unit.

Example: Unit of time is second.

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What is the need of measurement?

To get the complete knowledge of any physical quantity, measurement is needed. Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen, internationally accepted reference standard called unit. Knowledge without measurement is incomplete and unsatisfactory.
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