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Units and Measurement

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Physics Part I

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Class 10 Class 12
The length, breadth and thickness of a rectangular sheet of metal are 4.234 m, 1.005 m, and 2.01 cm respectively. Give the area and volume of the sheet to correct significant figures.


Length, l = 4.234 m 

Breadth,b = 1.005 m 

Thickness, t = 2.01 cm = 2.01 × 10-2 m

Area of the sheet = 2 (l × 0 + b × t + t × l)

                       = 2 (4.234 × 1.005 + 1.005 × 0.0201 + 0.0201 × 4.234)

                       = 2 (4.3604739)

                       = 8.7209478 m2

Area can contain a maximum of three significant digits, therefore, rounding off, we get

Area = 8.72 m2

Also, volume = l × b × t

               V = 4.234 × 1.005 × 0.0201

                  = 0.0855289

                  = 0.0855 m3 (Significant Figures = 3)



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. 

Give four examples of physical quantities.

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

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.