28: Power
- Page ID
- 91892
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Simply put, power is the rate of change of energy (or work) with time:
\[\mathcal{P}=\frac{d E}{d t}\]
In SI units, power is measured in units of watts (W), named for the Scottish engineer James Watt:
\[1 \mathrm{~W}=1 \frac{\mathrm{J}}{\mathrm{s}}=1 \frac{\mathrm{kg} \mathrm{m}^{2}}{\mathrm{~s}^{3}}\]
In CGS units, power is measured in units of statwatts:
\[1 \text { statwatt }=1 \frac{\mathrm{erg}}{\mathrm{s}}=1 \frac{\mathrm{g} \mathrm{cm}^{2}}{\mathrm{~s}^{3}}\]
The British engineering unit of power has no special name; it is simply a foot-pound per second (ft-lbf/sec).
Another common unit that is not part of the British engineering system is the horsepower (hp): \(1 \mathrm{hp}=\) \(550 \mathrm{ft}-\mathrm{lbf} / \mathrm{sec}\), or about 745.7 watts. The power produced by an automobile engine is traditionally measured in horsepower. A few examples:
- Lawn mower: \(5 \mathrm{hp}\)
- Smart car: \(90 \mathrm{hp}\)
- Typical modern automobile engine: about \(200 \mathrm{hp}\)
- 1967 Pontiac GTO “muscle car”: \(360 \mathrm{hp}\)
- Semi truck (tractor): \(500 \mathrm{hp}\)
- Modern farm tractor: \(500 \mathrm{hp}\)
- Formula One engine used in a modern Indianapolis 500 race car: \(700 \mathrm{hp}\) or more
- "Monster truck" (as seen at county fairs): \(1500 \mathrm{hp}\)