Mechanics is the science which treats of the nature of forces and of their action on bodies, either directly or by the agency of machinery. See FORCE, ENERGY, DYNAMICS. The action of forces on bodies may be in the form of pressure or of impulse, and may or may not produce motion. When the forces are so balanced as to preserve the body affected by them in a state of equilibrium, their actions are investigated in that branch of mechanics called Statics; when motion is produced, they are considered under the head of Kinetics (q.v.). See also the articles on Kinematics, Hydrostatics, Hydrodynamics, and Pneumatics.
Machines are instruments interposed between the moving power and the resistance, with a view of changing the direction of the force, or otherwise modifying it. Machines are of various degrees of complexity; but the simple parts, or elements of which they are all composed, are reducible to a very few. These elementary machines are called the Mechanical Powers, and are usually reckoned as six in number, three being primary—viz. the lever, inclined plane, and pulley; and three secondary, or derived from the others—viz. the wheel-and-axle (derived from the lever), the wedge, and the screw (both derived from the inclined plane). What is special to each machine will be found under its name.