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Physics · Glossary

What is Power?

Also known as: watt · power mechanical · horsepower

Definition 9.15 High School Physics · Chapter 9 — Energy: Forms and Conservation

Power is the rate at which energy flows, P=E/tP = E/t, measured in watts (W\mathrm{W}), with 1 W=1 J/s1\,\mathrm{W} = 1\,\mathrm{J}/\mathrm{s}. Energy is an amount, power a flow — a bathtub versus its tap.

Examples

Example 9.16 (Kettle versus human)

A 2200 W2200\,\mathrm{W} kettle running 150 s150\,\mathrm{s} uses E=2200×150=3.3×105 JE = 2200 \times 150 = 3.3 \times 10^{5}\,\mathrm{J}. A human runs on about 10 MJ10\,\mathrm{MJ} of food per day, an average 1.0×107/86 400≈116 W1.0 \times 10^{7}/86\,400 \approx 116\,\mathrm{W}: you idle like a bright old light bulb.

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Definition 17.13 High School Physics · Chapter 17 — Work of a Force

The power of a force is the rate at which it works, P=W/ΔtP = W/\Delta t, in watts (W=J/s\mathrm{W} = \mathrm{J}/\mathrm{s}) — the same watt as in Chapter 12. A pre-SI unit survives on engine badges: the horsepower, 1 hp=736 W1\,\mathrm{hp} = 736\,\mathrm{W}.

Examples

Example 17.15 (Crane)

A crane hoists a 600 kg600\,\mathrm{kg} pallet at a steady 0.90 m/s0.90\,\mathrm{m}/\mathrm{s}: the cable’s force equals the weight, mg=5.89×103 Nmg = 5.89 \times 10^{3}\,\mathrm{N}, so P=Fv=5890×0.90≈5.3 kW≈7.2 hpP = Fv = 5890 \times 0.90 \approx 5.3\,\mathrm{kW} \approx 7.2\,\mathrm{hp} — a seven-horse team in a steel box.

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