Power

Carry your bag up the stairs in one minute, then in five minutes. The work is the same (same bag, same height), but running feels much harder. The difference is power.

Power is how fast work is done: P = W/t (work ÷ time). Remember: "rate" means divided by time.

You need more power to:

• do more work in the same time; or
• do the same work in less time.

Two friends carry the same load up the same stairs; the one who gets there first has more power.

Power is measured in watts (W): 1 W = 1 J per second. It is named after James Watt, who invented an efficient steam engine. Engines and pumps are also rated in horsepower: 1 hp = 746 W.

A weightlifter raises 75 kg by 2 m in 5 s. W = 75 × 10 × 2 = 1500 J. P = 1500 ÷ 5 = 300 W.

A car of 1000 kg goes from rest to 72 km h⁻¹ (20 m s⁻¹) in 10 s. Its kinetic energy: W = ½ × 1000 × 20² = 200000 J. P = 200000 ÷ 10 = 20000 W.