# Measurement of Physical Quantities | Class 11

Measurement of any physical quantity involves comparison of its magnitude with a standard of the same kind. The standards are fixed arbitrarily, but there is always logic behind such as choice.

The arbitrarily fixed standard is called a unit. For example – to measure length a standard unit of metre is used and for measure mass standard unit kilogram is used.

Generally these standard units are prescribed by the International Bureau on Weight and Measures.

For example – a kilogram is defined as => The mass equal to the mass of a cylinder of platinum-irridium alloy kept at the International Bureau on Weights and Measures in Paris is called a kilogram.

Hence to express any measurement one should know following
1. Unit in which quantity is measured
2. Numerical value of measure

Unit is a standard, but Numerical value depends upon reliability of measuring instrument and judgement of person doing the measurement.

Thus in order to measure anything you should take multiple measurements and use average as the final Numerical value.

The limit of the reproducibility of any measurement is known as its precision. A measurement may be absolutely precise, or it may have some uncertainty depending upon the least-count of the instrument and the observation power of the person making measurement.

For example – an exact number like number of eggs, number of men etc. is absolutely precise but the measured mass of an object is not absolutely precise. The same object may weigh differently on different scales. The precision in the measurement of mass of an object depends upon.

Sensitivity of the weighing scale, observation power of the person making the measurement. The precision of any measurement is nothing to do with accuracy.

The degree of conformity of a measure to its true value is termed as its accuracy. Suppose that you used four weight measure scales and it shows that your weight is 40kg, 39kg, 41kg and 39kg. This makes you confused as what exactly is your weight as different machine is showing you differently.

Suppose that your weight actually is 40kg, then uncertainty in measurement of your weights by different machines is ± 1 kg. Thus you weight can be 40 ± 1 kg which means your weight is anywhere between 40 – 1 = 39 kg to 40 + 1 = 41 kg.