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Physics · Class 9 · Chapter 1: Introduction

8 min

Key points

Physics is the branch of science which deals with the study of matter, energy and their mutual relationship
Physical quantities are quantities which can be measured and have numerical values with proper units
Base quantities are fundamental quantities which cannot be expressed in terms of other quantities
Derived quantities are quantities which can be expressed in terms of base quantities
SI system has seven base units: meter, kilogram, second, ampere, kelvin, mole, candela
Significant figures are meaningful digits in a measured or calculated value
Scientific notation is used to express very large or very small numbers conveniently
Prefixes like micro, milli, kilo, mega are used to express multiples and sub-multiples of units

Definitions

Physics: Physics is the branch of science which deals with the study of matter, energy and their mutual relationship.
Physical Quantity: A physical quantity is a quantity which can be measured and has a numerical value with proper unit.
Base Quantity: Base quantities are fundamental quantities which cannot be expressed in terms of other physical quantities.
Derived Quantity: Derived quantities are quantities which can be expressed in terms of base quantities.
Unit: Unit is the standard amount of a physical quantity chosen to measure the physical quantity of the same kind.

Worked example

Define chemistry and classify matter on the basis of composition. Give one example of each type. (7 marks)

Chemistry is the branch of science which deals with the study of composition, structure and properties of matter and the changes it undergoes during chemical reactions. Matter can be classified as: (1) Pure Substances - Element: Oxygen (O₂), Compound: Water (H₂O). (2) Mixtures - Homogeneous: Sugar solution, Heterogeneous: Sand and water.

Common mistakes

Students confuse base quantities with derived quantities
Writing wrong SI units for physical quantities
Incorrect counting of significant figures, especially with zeros
Forgetting to write units in final answers
Mixing up prefixes and their numerical values (kilo = 10³, not 10²)

Quick recap

Chemistry studies composition, structure and properties of matter. Matter exists as pure substances (elements like O₂, compounds like H₂O) or mixtures (homogeneous like air, heterogeneous like oil-water). Physical properties can be observed without changing composition, chemical properties involve reactions. Scientific method involves observation, hypothesis, experimentation and conclusion.

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