Class 11 Physics Units and Measurements – Short Notes

Yashwant Parihar,

Units and Measurements is the first and most important step in learning Physics because every calculation, experiment, and formula is built on how accurately we measure the physical world around us.

In these Class 11 Physics short notes on Units and Measurements, you will find simple explanations of SI units, fundamental and derived quantities, dimensional analysis, significant figures, and types of errors, all aligned with the latest NCERT syllabus.

Difference Between Fundamental Quantity and Derived Quantity

Fundametal QuantityDerived Quantity
Fundamental quantities are those that do not depend on other physical quantities for their measurements. Derived quantities are those that can be expressed in terms of fundamental/base qunatities.
There are seven main fundamental quantities – Mass, Length, Time, Temperature, Current, Luminous Intensity and mole.All the quantities other than the seven fundamental quantities are known as derived quantities. For Example – Speed, Velocity, Force, Angle, etc.

System of Units

  1. FPS System: In this system, length is measured in feet, mass is measured in pounds, and time is measured in seconds.
  2. CGS System: In this system, length is measured in centimetres, mass is measured in grams, and time is measured in seconds.
  3. MKS System: In this system, length is measured in meters, mass is measured in kilograms, and time is measured in seconds.

Principle of Homogeneity

According to this principle, the physical quantities having the same dimensions can be added or subtracted from each other. If an equation is given, then the dimensions of each side must be the same.

For Example,

F=Am+Bv+CF = A \sqrt{m} + \frac {B}{v} + C

In the above equation, the dimension of force is equal to all three parts of the RHS.

What are Dimensions?

Dimension are representation of fundamental quantities with thier powers needed to express a physicsal quantity is known as dimensions.

Usage of Dimensional Analysis

  1. To check the correctness of formulas.
  2. To establish a relation between quantities dimensionally.
  3. To convert the quantity from one system to another system

Limitations of Dimensional Formula

1. It does not predict the numerical value associated with a physical quantity in a relation.

e.g,12mv2=mv2e.g, \frac {1}{2} mv^2 = mv^2

2. It does not derive the relations involving logarithms, exponential, and trigonometric functions.

3. We cannot find any information about dimensionally constant and the nature of quantity (Scalar and Vector).

Rules of Significant Figures or Digits

  1. All the non zero number are counted as significant. E.g. 28167 has 5 Significant Figures.
  2. If the zeros lie between the number they are also counted as significant. E.g. 820708 has 6 significant figures.
  3. All the zeros after the first non-zero digit are not considered significant. E.g. 0001201 has 4 significant figures.
  4. Trailing Zeros in Whole Numbers (Without Decimal Point) May or May Not Be Significant, e.g., 1500 may have 2, 3 or 4 significant figures to calculate it. Write in scientific notation 15 x 102 has 2 significant figures.

Errors in Measurement

Measurement is never perfectly accurate. The difference between the true value and the measured value is called error.

Types of errors:

  • Systematic error
  • Random error
  • Least count error
Physics Class 11 Short Notes Physics