Unit – I A

Measurement and its Need in Physics:

  • A measurement is a quantitative description of one or more fundamental properties compared to a standard.
  • The measurement of a quantity is mentioned in two parts, the first part gives how many times of the standard unit and the second part gives the name of unit. e.g. 5 m.

Need of Measurement:

  • Measurement is that operation by which we compare a physical quantity with an unit chosen for that quantity.
  • To study the properties of matter and energy and to verify the laws we have to perform experiments and we have to take reading. All these experiments require some measurements to be made. This clearly explains the need of measurements.
  • When we are producing the parts on machinery we have to measure the parts so as to find whether the part is made as per the specifications. Hence we can say that the measurements are necessary for production control.


Units of Measurements:

  • For measurements we must choose a standard or unit. For any measurement a number and unit is required.
  • When we say that the length of the rod is 5 metres, then it means that the length of the rod is 5 times a certain standard length called 1 metre.
  • A unit is a selected magnitude of physical variable in terms of which other magnitudes of same variable can be expressed.

Criteria of a Selection of Unit:

  • The selection of unit depends on the magnitude of a quantity under consideration. For e.g. when we are measuring the diameter of a rod we should use millimetre as a unit. When we are measuring the height of tower we should use metre as a unit. When we are measuring the distance between the two cities we should use kilometre as a unit. When we are measuring the distance between the two stars we should use light years as unit.
  • The accuracy of measurement also influence the selection of unit. In the case of construction of a room where accuracy is not a major criteria metre or foot are used as units. But when constructing a rocket accuracy is the criteria hence milimeter or micrometer may be the unit.
  • This explanation clearly indicates that as the magnitude of the measurement increases the unit used should be large. similarly the accuracy criteria increases the unit of measurement becomes smaller.


Properties of Units:

  • The standard unit must be invariable.
  • The standard unit should be easily available.

Requirements of Standard :

  • The standard unit should be easily available.
  • The standard should be non destructible
  • The standard should not change with the time
  • The standard should not change with the place
  • The standard should be easily reproducible

Characteristics of Standard Unit:

  • It should be well defined.
  • It should be of suitable size. i.e. neither too long nor to small in comparision with quantity to be measured.
  • It should be easily available.
  • It should be non destructible
  • It should not change with the time
  • It should not change with the place
  • It should be easily reproducible


Note:

  • A international body Conference Generale des Poids et Measures or CGPM (General Conference of Weight and Mesaures has been given the authority to decide the standards and units by international agrrement.
  • It holds its meetings and and any change in the standard units are communicated through the publications of the Conference.

Types of Physical Quantities:

  • Physical quantities are those quantities which are measurable. The Physical quantities are classified as
    a)  Fundamental quantities and  b) Derived quantities

    Fundamental Quantities:

    Fundamental quantities are those quantities which do not depend on other quantities for their measurements. The units of fundamental quantities are called as fundamental units.
    e.g. mass, length, time etc. are fundamental quantities.
    e.g. metre, kilogram, second etc. are fundamental units.

    Derived Quantities:

    Derived quantities are those quantities which depend on other quantities for their measurements. The units of derived quantities is called as derived units
    e.g. density, acceleration, velocity, force, momentum, pressure etc. are derived quantities.
    e.g. kg/m3, m/s2, m/s, newton, kg-m/s, pascal etc. are derived units.

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