Two equivalent definitions of “power”

Power = rate at which work is done

P = work / time

Power = rate that energy is transformed from one form to another

P = energy transformed / time

In honor of James Watt – 18th century developer of the steam engine – this unit has been named the watt.

(name of the unit is lowercase, but the abbreviation is uppercase.)

unit of power = 1 J / s = 1 W


We often use large units of power

1 kilowatt = 1000 watts

1 megawatt = 1,000,000 watts


Scottish engineer James Watt to compared the output of steam engines with the power of draft horses. 18th cent

draft horse: a large horse bred to be a working animal doing hard tasks such as plowing and other farm labor. (Wikipedia)

Draft horse

Watt determined that a horse could do 33,000 foot-pounds of work in a minute:

move 330 pounds of coal 100 feet in a minute,
or haul 33 pounds of coal 1000 feet in the same time
He dubbed that “one horsepower.”


In the British system the unit of power is the foot-pound/second, or the horsepower.

Horsepower = 746 W


Examples of engines doing work




The atmospheric engine invented by Thomas Newcomen in 1712.

1st practical device to harness steampower for mechanical work.

Used throughout Britain and Europe, principally to pump water out of mines. Hundreds were constructed through the 18th century.

James Watt’s later engine design was an improved version, that doubled fuel efficiency. Many atmospheric engines were converted to the Watt design. Watt is today better known than Newcomen in relation to the origin of the steam engine.

(adapted from Wikipedia, https://en.wikipedia.org/wiki/Newcomen_atmospheric_engine)

Watt Steam engine

Explanation of how this steam engine works


Example: Power to climb a flight of stairs.

Work is related to a change in potential energy




Learning Standards

Massachusetts Science and Technology/Engineering Curriculum Framework

2. Conservation of Energy and Momentum
2.3 Describe both qualitatively and quantitatively how work can be expressed as a change in mechanical energy.
2.4 Describe both qualitatively and quantitatively the concept of power as work done per unit time.

SAT Physics Subject Test Learning Standards

Mechanics: Energy and momentum, such as potential and kinetic energy, work, power, impulse, and conservation laws


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