Work, in physics, is a result of a force moving an object through a distance against a resistance. Work is always done by some agent, such as a person or a machine. This agent produces the force that causes the movement.
Two factors determine the amount of work done. One factor is the amount of force applied. The other is the distance the object moves. In physics, work occurs only when the force is sufficient to move the object. In other words, work is a measure of what is done, not the effort applied in attempting to move the object. People do work when lifting, pushing, or sliding an object from one place to another. They do no work when holding an object without moving it, even though they may become tired.
People also can do work when rotating an object. For example, a person who unscrews the lid of a milk jug does work. In this case, the resistance is the force of friction that the screw threads of the jug exert against the lid.
Scientists and engineers measure work in units that represent the measurement of both force and distance. In the inch-pound system of measurement customarily used in the United States, the foot-pound is the most commonly used unit of work. One foot-pound equals the work done when a force of 1 pound moves something a distance of 1 foot. So if a 50-pound object is lifted 4 feet, the work done is 200 foot-pounds. If a 4-pound object is lifted 50 feet, the work done is also 200 foot-pounds.
In the metric system, the unit of work is the joule. One joule is the amount of work done when a force of 1 newton moves something a distance of 1 meter (see Newton ). Thus, 1 joule equals 1 newton-meter. One foot-pound equals 1.356 joules.
The calculation of work done during rotation is more complicated. However, this kind of calculation uses the same principles used to compute the amount of work done when an object is moved in a straight line.
The units used to measure work are also used to measure energy. Energy is the ability of something to do work (see Energy ). The rate at which work is done is called power. In measuring power, the amount of time needed to do the work is considered along with force and distance. Power is measured in a unit called the watt. One watt equals one joule per second. See Power ; Watt .
