Units & Measurements — The Language of Physics

Fundamental vs derived quantities, the 7 SI base units, dimensional formulas, and significant figures.

By the PadhoDost Team · 📖 6 min read · Updated 4 August 2026

Part of JEE (Main + Adv) prep

🧠 Measuring = comparing

Every measurement is a comparison against a standard. Saying a table is '2 metres' means it's twice the agreed 'metre'. Physics just needs everyone to agree on the standards — that's what units are.

Fundamental vs derived

A few quantities are FUNDAMENTAL (base) — defined on their own. Everything else is DERIVED by combining them. e.g. speed = length ÷ time is derived from length and time.

The 7 SI base units

  • ✓metre (m) — length
  • ✓kilogram (kg) — mass
  • ✓second (s) — time
  • ✓ampere (A) — electric current
  • ✓kelvin (K) — temperature
  • ✓mole (mol) — amount of substance
  • ✓candela (cd) — luminous intensity

Dimensional formulas

A dimensional formula shows which base quantities make up a derived one, using M (mass), L (length) and T (time).

velocity = [LT⁻¹] acceleration = [LT⁻²] force = [MLT⁻²]
💡 Significant figures: leading zeros never count, but trailing zeros after a decimal do. So 0.00450 has 3 significant figures (4, 5 and the last 0).

⚡ Quick check

How many fundamental (base) units are there in the SI system?

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