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Units and Measurement

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Physics Part I

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Class 10 Class 12

Just as precise measurements are necessary in science, it is equally important to be able to make rough estimates of quantities using rudimentary ideas and common observations. Think of ways by which you can estimate the following (where an estimate is difficult to obtain, try to get an upper bound on the quantity):

(e) the number of air molecules in your classroom.

Let, the volume of the classroom be V.

One mole of air at NTP occupies 22.4 l.
i.e., 22.4 × 10–3 m3volume.

Number of molecules in one mole = 6.023 × 1023 

∴ Number of molecules in room of volume V,

= 6.023 × 1023 × V / 22.4 × 10-3 

=  134.915 × 1026 V 

=  1.35 × 1028 V 


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. 

Define unit.

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

Example: Unit of time is second. 


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. 

Give four examples of physical quantities.

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

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.