Engine Engine An engine or motor is a machine designed to convert one form of energy into mechanical energy. Heat engines burn a fuel to create heat which is then used to do work. Internal combustion engines are heat engines that burn fuel in a combustion chamber to extract work from the pressure of expanding gases. Electric motors convert electrical energy into mechanical motion; pneumatic motors use compressed air; and clockwork motors in wind-up toys use elastic energy. In biological systems, molecular motors, like myosins in muscles, use chemical energy to create forces and eventually motion. The word "engine" derives from
Potential energy the gravitational potential, often denoted by formula_47 or formula_48, corresponding to the energy per unit mass as a function of position. The gravitational potential energy of two particles of mass "M" and "m" separated by a distance "r" is The gravitational potential (specific energy) of the two bodies is where formula_51 is the reduced mass. The work done against gravity by moving an infinitesimal mass from point A with formula_52 to point B with formula_53 is formula_54 and the work done going back the other way is formula_55 so that the total work done in moving from A to B