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Work equals force times distance through which the force works.

For scientists, power is the rate at which work is performed by a force acting along a distance.^M As an example shown on the slide, the Wright 1903 aircraft is acted upon by the thrust force F from time t equals zero to some later time t > 0 and travels some distance s. The work W done on the aircraft during this time is F times s.

W = F * s

The average power P expended by the engine to perform this work is equal to the work divided by the time.

P = W / t

P = F * s / t

The unit of power in the metric system is the watt, which is equal to one joule per second. In the English system the unit of power is the horsepower hp which is equal to 550 foot-pounds per second. In our simple example, the force is a constant value aligned with the displacement of the aircraft and the velocity V is constant. The power then becomes the product of the force and the velocity:

P = F * V

The Wright aircraft developed 130 pounds of thrust and required 32 miles per hour airspeed to fly. The motor was then developing about 11 horsepower: [130 pounds times 32 mph (converted to feet per second 88 fps = 60 mph) divided by 550 foot-pounds per second per horsepower = 11.09 hp] For comparison, a modern lawn mower engine is rated at 6 horsepower. The Wright brothers used their knowledge of work and power to determine the minimum requirements for their engine. They were able to make rather accurate estimates of the drag of their aircraft based on wind tunnel tests and flight tests of their earlier gliders. Knowing the drag and the desired flight velocity, they computed the power requirements for the engine needed to develop enough thrust to overcome the drag.


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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 05 2015

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