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Units and Measurement

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Physics Part I

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Class 10 Class 12
The principle of ‘parallax’ in section 2.3.1 is used in the determination of distances of very distant stars. The baseline AB is the line joining the Earth’s two locations six months apart in its orbit around the Sun. That is, the baseline is about the diameter of the Earth’s orbit 3 × 1011m. However, even the nearest stars are so distant that with such a long baseline, they show parallax only of the order of 1” (second) of arc or so. A parsec is a convenient unit of length on the astronomical scale. It is the distance of an object that will show a parallax of 1” (second) of arc from opposite ends of a baseline equal to the distance from the Earth to the Sun. How much is a parsec in terms of metres ?

Diameter of Earth’s orbit = 3 × 1011 m

Radius of Earth’s orbit, r = 1.5 × 1011 m

Let the distance parallax angle be 1" = 4.847 × 10–6 rad 

Let the distance of the star be D.

Parsec is defined as the distance at which the average radius of the Earth’s orbit subtends an angle of 1"

Therefore comma space

we space have space straight theta space equals space straight r over straight D space

rightwards double arrow space straight D space equals space straight r over straight theta space equals space fraction numerator 1.5 cross times space 10 to the power of 11 over denominator 4.847 space cross times space 10 to the power of negative 6 end exponent end fraction space

space space space space space space space space space equals space space 0.309 space cross times space 10 to the power of negative 6 space end exponent almost equal to space 3.09 space cross times space 10 to the power of 16 space straight m space


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.


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. 

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 unit.

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

Example: Unit of time is second.