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Units and Measurement

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Physics Part I

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Class 10 Class 12
A SONAR (sound navigation and ranging) uses ultrasonic waves to detect and locate
objects under water. In a submarine equipped with a SONAR the time delay between generation of a probe wave and the reception of its echo after reflection from an enemy submarine is found to be 77.0 s. What is the distance of the enemy submarine?
(Speed of sound in water = 1450 m s –1).

Let the distance between the ship and the enemy submarine be ‘S’.

Speed space of space sound space in space water space equals space 1450 space straight m divided by straight s space

Time space lag space between space transmssion space and
reception space of space Sonar space waves space equals space 77 space straight s

In space this space time space lag comma space
sound space waves space travel space straight a space distance space which space is space twice
space the space distance space between space the space ship space and space the space submarine space left parenthesis 2 straight S right parenthesis.

Time space taken space for space the space sound space to space
reach space the space submarine space equals space 1 divided by 2 space cross times space 77 space equals space 38.5 space straight s space

Therefore comma space

Distance space between space the space ship space and space the space submarine comma space

left parenthesis straight S right parenthesis space equals space 1450 space cross times space 38.5 space

space space space space space space equals space 55825 space straight m space equals space 55.8 space km

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 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. 

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.