Units and Measurement

Physics Part I

Physics

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A famous relation in physics relates ‘moving mass’ *m *to the ‘rest mass’ *m*_{o}of a particle in terms of its speed *v *and the speed of light, *c*. (This relation first arose as

a consequence of special relativity due to Albert Einstein). A boy recalls the relation

almost correctly but forgets where to put the constant c. He writes :

Guess where to put the missing*c.*

a consequence of special relativity due to Albert Einstein). A boy recalls the relation

almost correctly but forgets where to put the constant c. He writes :

Guess where to put the missing

Given the relation,

Dimension of m = M^{1} L^{0} T^{0}Dimension of m_{o} = M^{1} L^{0} T^{0 }Dimension of v = M^{0} L^{1} T^{–1}Dimension of v^{2} = M^{0} L^{2} T^{–2}Dimension of c = M^{0} L^{1} T^{–1}

The given formula will be dimensionally correct only when the dimension of L.H.S is the same as that of R.H.S.

This is possible when the factor in the denominator is dimensionless. This is only possible if

Hence the correct relation is

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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.

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Define unit.

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

Example: Unit of time is second.

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Give four examples of physical quantities.

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

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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.

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