What is a derived unit? Derive the SI unit of power in base units and give its special name.

1. 1 · Define. A derived unit is built from base units through the formula of the quantity.
2. 2 · Start from work. Work = force × distance = mass × acceleration × distance.
3. 3 · Unit of work. kg × m s⁻² × m = kg m² s⁻², the joule (J).
4. 4 · Unit of power. Power = work ÷ time, so its unit = kg m² s⁻² ÷ s = kg m² s⁻³.
5. 5 · Name it. This is the watt (W): 1 W = 1 J s⁻¹.
6. 6 · Real life. A 9 W LED bulb uses 9 J of energy every second.
7. 7 · Close. So the watt is a derived unit with a special name.

Scoring tip: Show each line of units; the final unit alone earns only part of the marks.