A machine is any device that transmits mechanical energy or power into useful work. The simplest machines redirect an applied force or convert kinetic energy into potential energy. More complicated machines are powered by electricity or fuel and use moving parts and electronic circuitry to complete complex tasks.
Machines range in sophistication from simple to incredibly complex. Inclined planes, wedges, levers and screws are examples of basic machines. They consist of one piece, and allow the user to transmit or enhance an applied force in a useful way. In the case of a screw, the twisting force applied to the screw allows the user to drive one solid substance into another. An inclined plane, on the other hand, transmits the kinetic energy used to roll an object uphill into potential energy.
Combustion engines use the power of a controlled chemical explosion to move certain mechanical parts in prescribed ways. In most engines, the power of fuel combustion forces a piston to move up and down in a chamber. The piston is connected to a series of other parts that transmit this motion into the spinning of an axle or gear. Electric machines use components such as resistors and diodes to direct electricity through circuits in a specific way. The electricity is then converted by other components into sound, light or other useful electromagnetic signals.