Define power and give its SI unit. A boy of mass 50 kg climbs 30 steps, each 15 cm high, in 15 s. Find his power. (g = 9.8 m s⁻²)

1. 1 · Define. Power is the rate of doing work: P = W/t.
2. 2 · Give the units. The watt (W): 1 W = 1 J s⁻¹. Also 1 kW = 1000 W and 1 hp = 746 W.
3. 3 · Find the height. h = 30 × 15 cm = 450 cm = 4.5 m.
4. 4 · Find the work. W = mgh = 50 × 9.8 × 4.5 = 2205 J.
5. 5 · Find the power. P = W/t = 2205 J ÷ 15 s = 147 W.
6. 6 · Close. So the boy's power is 147 W. Running up faster would raise it.

Scoring tip: Change cm to m before using mgh. Find the work first, then divide by the time.