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Units and Measurement

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Physics Part I

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CBSE Gujarat Board Haryana Board

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Class 10 Class 12
Explain this statement clearly : 
“To call a dimensional quantity ‘large’ or ‘small’ is meaningless without specifying a
standard for comparison”. In view of this, reframe the following statements wherever
necessary : 
(a) atoms are very small objects 
(b) a jet plane moves with great speed 
(c) the mass of Jupiter is very large 
(d) the air inside this room contains a large number of molecules 
(e) a proton is much more massive than an electron 
(f) the speed of sound is much smaller than the speed of light

The given statement is true because a dimensionless quantity may be large or small in comparison to some standard reference. For example, the coefficient of friction is dimensionless. The coefficient of sliding friction is greater than the coefficient of rolling friction, but less than static friction. 

a) An atom is a very small object in comparison to a soccer ball.

b) A jet plane moves with a speed greater than that of a bicycle.

c)  Mass of Jupiter is very large as compared to the mass of a cricket ball.

d)  The air inside this room contains a large number of molecules as compared to that present in a geometry box.

e) A proton is more massive than an electron.

f) Speed of sound is less than the speed of light.


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. 

Define unit.

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

Example: Unit of time is second. 


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.


Give four examples of physical quantities.

Examples of Physical quantitie are force, mass, density and power.

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.