Electricity also powers the levers that help open and shut the elevator doors. Scientists find the mechanical advantage of a machine by dividing the force the machine delivers by the effort put into the machine. The theoretical, or ideal, mechanical advantage of a machine is the advantage it would produce if the machine were perfect. In simple machines, the main source of imperfection is friction . Friction results from two bodies moving against each other in different directions.
Friction always opposes motion and makes doing work harder.
Since friction is present in almost every machine, the actual mechanical advantage is always less than the theoretical mechanical advantage. Because simple machines increase mechanical advantage by increasing the distance over which the effort is applied, one way to compute theoretical mechanical advantage is to divide the distance the effort is applied by the distance the load actually travels. This means that the work was twice as easy, or that only half as much effort was needed to raise the load. Because of the inclined plane, however, the load needed to be pushed twice as far to end up 5 meters above the ground. Another factor that people sometimes compute for machines is their efficiency , or the ratio of the work that results to the amount of work put into the machine.