Mechanical engineering is the application of physical principles to the creation of useful devices, objects and machines. Mechanical engineers use principles such as heat, force, and the conservation of mass and energy to analyze static and dynamic physical systems, in contributing to the design of things such as automobiles, aircraft, and other vehicles, heating and cooling systems, household appliances, industrial equipment and machinery, weapons systems, etc.
Mechanical engineers often create simulations of the operation of objects, as well as the manufacturing processes to be used, in order to optimize performance, cost effectiveness, and energy efficiencies, before settling on a particular design.
Engineering drawings of the objects to be fabricated are typically created. Prior to the late 20th Century most engineering drawings were drawn by hand with the aid of mechanical drafting boards. The advent of the digital computer with graphical user interface made the creation of models and drawings using Computer Aided Design (CAD) programs possible. Most CAD programs now permit creation of three-dimensional models which may be viewed from any angle. Such models may be used as the basis for Finite Element Analysis (FEA) and / or Computational Fluid Dynamics (CFD) of the design. Through the application of Computer Aided Manufacturing (CAM), the models may also be used directly by software to create "instructions" for the manufacture of objects represented by the models, through computer numerically-controlled (CNC) machining or other automated processes, without the need for intermediate drawings.
Fundamental subjects of mechanical engineering include: dynamics, statics, strength of materials, heat transfer, fluid dynamics, solid mechanics, control theory, pneumatics, hydraulics, mechatronics and applied thermodynamics. Mechanical engineers are also expected to understand and be able to apply concepts from the chemistry and electrical engineering fields. At the smallest scales, mechanical engineering becomes molecular engineering - one speculative goal of which is to create a molecular assembler to build molecules and materials via mechanosynthesis. For now this goal remains within exploratory engineering, and some consider it science fiction.