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Physics · Class 9 · Chapter 2: Measurement

8 min

Key points

Physics is a quantitative science — all measurements require magnitude and unit
SI System has 7 base units: metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), candela (cd)
Length is measured using metre rule, vernier callipers, and screw gauge for different precisions
Vernier callipers can measure up to 0.1 mm accuracy, screw gauge up to 0.01 mm accuracy
Least count = Smallest division on main scale ÷ Total number of divisions on vernier scale
Mass is measured using physical balance, electronic balance, and beam balance
Time is measured using stopwatch, pendulum clock, and atomic clock
Significant figures follow specific rules — all non-zero digits are significant
Scientific notation is used to express very large or very small numbers
Accuracy refers to closeness to true value, precision refers to consistency of measurements

Definitions

Measurement: Comparison of an unknown quantity with some known fixed quantity of the same kind called unit
Unit: A definite amount of a physical quantity taken as a standard for measurement of that quantity
Least Count: The smallest measurement that can be taken accurately with a measuring instrument
Vernier Callipers: An instrument used to measure internal and external diameters of tubes and thickness of sheets accurately
Significant Figures: The number of digits in a measurement that are known with certainty plus one digit that is uncertain

Worked example

Define measurement and explain the difference between base quantities and derived quantities. Give two examples of each. (5 marks)

Measurement is the comparison of an unknown quantity with a known standard quantity called unit. Base quantities are those physical quantities which are independent and cannot be expressed in terms of other physical quantities. Examples: length, mass. Derived quantities are those physical quantities which are expressed in terms of base quantities. Examples: area, velocity.

Common mistakes

Students often forget that zeros before decimal are not significant figures
Confusing least count of vernier callipers (0.1 mm) with screw gauge (0.01 mm)
Not writing proper units with final answers in numerical problems
Mixing up main scale reading with vernier scale reading in callipers
Forgetting to add MSR + (VSD × LC) formula in vernier callipers problems

Quick recap

Measurement compares unknown quantities with standard units. Physical quantities are base (independent like length, mass, time) or derived (dependent like area, speed). SI system has 7 base units. Measuring instruments have least counts that determine precision.

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