Measurement and Measuring Instruments


  • A unit is the standard quantity with which unknown quantities are compared.
  • For example Kilogram is the unit of measuring Weight.

SI system of Units

Derived Quantities and their Units

Fundamental units of SI system


  •       Length is defined as the distance between two points.
  •       The SI unit of length is meter.
  •       One meter is the distance traveled by light through vacuum in 1/29,97,92,458 second.

Light year: It is the distance traveled by light in one year in vacuum and it is equal to 9.46 × 1015 m.

Astronomical unit (AU): It is the mean distance of the center of the Sun from the  center of the earth. 1 AU = 1.496 × 10 11 m.

Parsec: Parsec is the unit of distance used to measure astronomical objects outside the solar system.

1 parsec = 3.26 light year.


  • Mass is the quantity of matter contained in a body.
  • The SI unit of mass is kilogram.

1 quintal = 100 kg

1 metric tonne = 1000 kg = 10 quintal

1 solar mass = 2 × 10 30 kg

  • Mass of a proton, neutron and electron can be determined using atomic mass unit.
  • 1 amu = 1/12th of the mass of carbon-12 atom.


  • Time is a measure of duration of events and the intervals between them.
  •  The SI unit of time is second.
  • One second is the time required for the light to propagate 29,97,92,458 metres through vacuum.
  • It is also defined as 1/86, 400th part of a mean solar day.
  • A chronon is a proposed quantum of time, that is, a discrete and indivisible “unit” of time as part of a hypothesis that proposes that time is not continuous.


  • Temperature is the measure of hotness. SI unit of temperature is kelvin(K).
  • One kelvin is the fraction of 1/273.16 of the thermodynamic temperature of the triple point of water .
  • Zero kelvin (0 K) is commonly known as absolute zero.
  • The other units for measuring temperature are degree Celsius and Fahrenheit .
  • The formula for converting Celsius to Fahrenheit is (0°C × 9/5) + 32 = 32°F

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