A machine is a device that performs useful work by transmitting, modifying, or transforming motion, forces, and energy. There are three basic machines, the inclined plane, the lever, and the wheel and axle; from these, and adaptations of these, are built up all true machines, no matter how complex they may appear.


There are two essential properties of all machines: mechanical advantage, which is the ratio load/effort, and efficiency, the ratio of actual performance to theoretical performance. Mechanical advantage can be less than, equal to, or greater than 1; while efficiency, owing to such losses as friction, is always less than 100% (otherwise a perpetual motion machine would be possible).


Simple machines derived from the three basic elements include the inclined plane, the wedge (effort at the top being translated to force at the sides), and the screw, (an inclined plane in spiral form); from the lever, the wrench or spanner (the balance also uses the principle of the lever), and from the wheel and axle, the pulley, (which can also be viewed as a type of lever).