Infinity Learning

Lesson 3 — Units (SI, Derived, Prefixes)

Units tell us what a number measures. In physics we use the SI system: 7 base units build all others. Treat units like algebra—multiply, divide, square, and cancel them.

SI Base Units (7)

m (length) kg (mass) s (time) A (current) K (temperature) mol (amount) cd (luminous intensity)

SI Prefixes

k = 10³ M = 10⁶ G = 10⁹ c = 10⁻² m = 10⁻³ μ = 10⁻⁶ n = 10⁻⁹

Common Derived Units

N = kg·m/s² J = N·m = kg·m²/s² W = J/s = kg·m²/s³ Pa = N/m² = kg/(m·s²) Hz = s⁻¹ C = A·s V = W/A = kg·m²/(s³·A)

Unit Algebra

  • (m/s)² = m²/s² (exponents distribute).
  • Use dots (·) for multiply, slashes (/) for divide.
  • Write quantities with units and round at the end.

Try it

Variant 1 of many

Which is a base SI unit?

More practice (unlocks as you answer correctly)

Worked examples

Example 1 — Force: N = kg·m/s² by definition, so kg·m/s² is the Newton.

Example 2 — Pressure: Pa = N/m² = kg/(m·s²) because N = kg·m/s².

Example 3 — Prefix sense: 1 mm = 1000 μm ⇒ 1 mm is larger than 1 μm.

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