A complex machine, such as an airplane engine or an elevator, is made up of many simple machines. Airplane engines and elevators are not powered by hand. Complex machines often use the energy stored in chemical substances, such as airplane fuel or the energy stored in electricity, to provide the necessary force to do work. An airplane engine uses the combustion, or rapid burning, of airplane fuel to power the engine that turns the propeller. An elevator uses large engines, usually powered by electricity, to pull cables that raise and lower the elevator car.
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.