Standard Units

📚 Key Concepts

🔹 International System of Units (SI)

The SI System:

  • SI stands for Système International d’Unités (International System of Units)
  • Adopted by countries worldwide for standardization
  • Ensures measurements are consistent globally
  • Based on seven fundamental units

🔹 SI Unit of Length

Metre (m):

  • SI unit of length
  • Symbol: m (lowercase, no plural ‘s’, no full stop)
  • Definition: Originally defined as one ten-millionth of the distance from equator to North Pole
  • Modern definition: Length of path travelled by light in vacuum in 1/299,792,458 seconds

🔹 Units of Length – Conversion

Common Length Units:

Larger to Smaller:

  • 1 kilometre (km) = 1000 metres (m)
  • 1 metre (m) = 100 centimetres (cm)
  • 1 centimetre (cm) = 10 millimetres (mm)

Complete Conversion Chain:

1 km = 1000 m = 100,000 cm = 1,000,000 mm

When to Use Which Unit:

  • Kilometres (km): Long distances (between cities, countries)
  • Metres (m): Room dimensions, height of buildings, sports tracks
  • Centimetres (cm): Length of books, pencils, human height
  • Millimetres (mm): Thickness of coins, paper, small precise measurements

🔹 Other Historical Units

Imperial System (still used in some countries):

  • 1 inch = 2.54 cm
  • 1 foot = 12 inches = 30.48 cm
  • 1 yard = 3 feet = 91.44 cm
  • 1 mile = 1760 yards = 1.609 km
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