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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
A LASER is a source of very intense, monochromatic, and unidirectional beam of light. These properties of a laser light can be exploited to measure long distances.
The distance of the Moon from the Earth has been already determined very precisely using a laser as a source of light. A laser light beamed at the Moon takes 2.56 s to return after reflection at the Moon’s surface. How much is the radius of the lunar orbit around the Earth?

Radius of the lunar orbit is the distance between the earth and the moon. 

Laser beam takes 2.56 second to reflect back, therefore distance of the moon from the earth is given by, 

straight d space equals space 1 half space ct space
space space space equals space 1 half cross times 3 cross times 10 to the power of 8 cross times 2.56 space straight m

space space space equals space 3.84 space cross times 10 to the power of 8 space straight m


Define unit.

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

Example: Unit of time is second. 


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 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 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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