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Measurement/Unit Notes with PDF

By October 4, 2025 Educational Resources 0 Comments

These notes are your ultimate revision weapon to revise Measurement/Unit. We’ve distilled years of previous exam questions (PYQs) into one powerful, concise resource. Everything you need to know, nothing you don’t.

  • PYQs, Decoded: All key concepts from past exams, organized and simplified.
  • Revise in Record Time: Short, precise, and designed for last-minute review.
  • Focus on What Matters: Master high-probability topics and boost your confidence.
  • Free PDF to download.

Units of Measurement and Physical Quantities

I. Fundamental Concepts

  • Fundamental (Base) Quantities
    • There are seven fundamental physical quantities in the SI system.
    • They are: length, mass, time, electric current, temperature, light intensity, and amount of a substance.
    • Electric current is a fundamental quantity, measured in Amperes (A).
  • Derived Quantities
    • These are defined in terms of the fundamental quantities.
    • Examples include force, work, power, pressure, etc.

II. Mechanics

  • Force
    • Newton (N) is the SI unit of force. (1 N = 1 kg·m/s²).
    • Dyne is a unit of force in the older CGS system, not the SI system.
  • Work and Energy
    • Joule (J) is the SI unit of work and energy. 1 Joule = 1 Newton-meter.
    • Erg is a unit of energy in the CGS system.
  • Power
    • Watt (W) is the SI unit of power. 1 Watt = 1 Joule per second.
    • Megawatt (MW) is a larger unit of power used for power plants.
    • Horsepower (hp) is a unit of power. 1 horsepower = 746 watts.
  • Impulse
    • Measured in Newton-second (N·s).
  • Acceleration
    • Its SI unit is metres per second squared (m/s²).

III. Electricity and Magnetism

  • Electric Current
    • Measured in Amperes (A).
  • Electric Potential
    • Measured in Volts (V).
  • Resistivity
    • It is a material’s property to resist electric current flow.
    • Its unit is the Ohm-meter (Ω·m).
  • Electric Power
    • Measured in Watts (W), representing the rate of electrical energy transfer.

IV. Thermodynamics

  • Heat Energy
    • Measured in Calories (cal), Kilocalories (kcal), Joules (J), or Kilojoules (kJ).
    • Watt is a unit of power, not a unit of heat.
  • Temperature
    • Celsius (°C) is a scale for measuring temperature, not a unit of heat.

V. Wave Phenomena, Sound, and Light

  • Frequency
    • The unit of frequency is the Hertz (Hz), which equals one cycle per second.
  • Sound
    • Decibel (dB) is a unit used to measure sound intensity.
  • Light Wavelength
    • Angstrom (Å) is a unit of length used for light wavelengths. 1 Å = 10⁻¹⁰ meters.

VI. Pressure and Stress

  • Pressure
    • Pascal (Pa) is the SI unit of pressure. 1 Pascal = 1 Newton per square metre (N/m²).
    • Bar is a metric unit of pressure. 1 bar = 10⁵ Pascals (Pa). It is not an SI unit.
    • kgf/cm² (kilogram-force per square centimeter) is a unit for pressure. 1 kgf/cm² is approximately equal to 1 bar.
    • The expression kg/cm² is a deprecated (outdated) unit; the correct notation is kgf/cm².
  • Stress
    • Like pressure, it is measured in Pascals (Pa).

VII. Distance and Length

  • Astronomical Distances
    • Light-year: A unit of distance, not time. It is the distance light travels in one year in a vacuum (≈ 9.46 trillion km). It is used because the speed of light is constant.
    • Parsec: Another unit for large astronomical distances. 1 parsec ≈ 3.26 light-years.
  • Small Scales
    • Micron (Micrometer): 1 μm = 10⁻⁶ meters = 10⁻⁴ centimeters.
    • Nanometre (nm): 1 nm = 10⁻⁹ meters = 10⁻⁷ centimeters.
    • Angstrom (Å): 1 Å = 10⁻¹⁰ meters.
    • Fermi (Femtometer): Equal to 10⁻¹⁵ meters. It is the smallest unit of length listed.
  • Navigation
    • Nautical mile: A unit of distance used in navigation.
  • Conversions
    • 1 centimeter (cm) ≈ 0.39 inches. (1 inch = 2.54 cm).
    • 1 foot = 12 inches ≈ 0.305 meters.
    • 1 kilometre (km) = 1000 metres (m).

VIII. Speed

  • Knot: A unit of speed used for ships and aircraft (nautical miles per hour).
  • Mach number: A dimensionless quantity representing the ratio of an object’s speed to the speed of sound. Used to measure high speed.

IX. Mass

  • Kilogram (kg) is the SI unit of mass.
  • Pikogram (pg) is a unit of mass. 1 pikogram = 10⁻¹² grams.

X. Volume and Flow

  • Volume
    • 1 barrel of oil is equal to:
      • Approximately 159 liters.
      • 0.158987 cubic metres.
      • 42 U.S. gallons.
      • 35 U.K. gallons.
  • Flow Rate
    • Cusec: A unit for the rate of flow of water. It stands for cubic feet per second and is equivalent to 28.317 litres per second.

XI. Specialized Units

  • Earth Science
    • Richter scale: Measures the intensity (magnitude) of an earthquake.
    • Dobson Unit (DU): Measures the thickness of the ozone layer. 1 DU = 0.01 mm at standard temperature and pressure. Named after G.M.B. Dobson.
  • Technology & Computing
    • Byte: A unit of digital information, typically consisting of 8 bits.
  • Material Science
    • Strain: A measure of deformation. It is a ratio and is dimensionless (has no units).

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