The use of machines has enabled man to do work that he lacks the power to do unaided. Machines have also made it possible for him to harness the forces in the wind, in fuels, and in water. Without machines man would still exist in a primitive state and the progress he has made could never have come about. A machine is any device used to increase force, change the direction of force, or increase speed in performing work. Work is done only if something is moved by overcoming a resistance, such as friction or gravity. A simple machine has no energy source within it, so it cannot do work unless work is put into it.
Where friction causes only negligible loss of energy, work produced by a machine equals the amount of work put into it. This work of machines is measurable. It is the product of the unit of force and distance. For example, if a person lifts a ten pound box three feet, he has done three times ten pounds or thirty foot-pounds of work.
The mechanical advantage of a machine is the ratio of resistance to effort. For example, a man lifts fifty pounds of weight by applying ten pounds of effort to a lever. Then the mechanical advantage of the lever is five to one.
