Work, Power, and Energy in Physics

Recognize everyday words that have exact, measurable definitions in physics.

Physics borrows ordinary words and gives them strict definitions. In physics, 'work' is done only when a force moves an object through a distance in the direction of the force. Holding a heavy box perfectly still may be exhausting, but in physics it does no work on the box, because the box does not move. 'Energy' is the capacity to do work, measured in joules. 'Power' is the rate at which work is done or energy is transferred, measured in watts; one watt is one joule per second.

Worked example: two students carry identical boxes up the same stairs, one in 10 seconds and one in 20 seconds. They do the same work, because force and distance are equal. The faster student uses more power, because the same work happens in less time. Reading physics problems correctly depends on hearing these words in their technical sense.

• 1Work requires a force moving an object through a distance.
• 2Energy is the capacity to do work, in joules.
• 3Power is the rate of doing work, in watts.