Mechanics

Mechanics is the branch of physics that studies how force, matter, and motion interact in physical objects. When forces act on objects, they can cause displacements—meaning the object shifts its position relative to its surroundings. The theoretical foundations of this field trace back to ancient Greece, with early ideas from thinkers like Aristotle and Archimedes.

During the early modern period, scientists such as Galileo Galilei, Johannes Kepler, Christiaan Huygens, and Isaac Newton built the framework now known as classical mechanics. In the 20th century, new discoveries challenged classical concepts, leading to entirely new approaches: relativistic mechanics and quantum mechanics.

**History**

**Antiquity** Ancient Greek philosophers were among the first to suggest that abstract principles govern nature. The dominant theory in antiquity was Aristotelian mechanics, though an alternative appears in the pseudo-Aristotelian *Mechanical Problems*, often credited to one of Aristotle’s successors. Another Greek tradition used mathematics more extensively to analyze bodies in static or dynamic situations, possibly inspired by the Pythagorean Archytas. Examples include pseudo-Euclid’s *On the Balance*, Archimedes’ *On the Equilibrium of Planes* and *On Floating Bodies*, Hero’s *Mechanica*, and Pappus’s *Collection* (Book VIII).

**Medieval Age** In the Middle Ages, Aristotle’s theories faced criticism and revision, starting with John Philoponus in the 6th century. A key issue was projectile motion, discussed by Hipparchus and Philoponus. The Persian Islamic scholar Ibn Sīnā, in *The Book of Healing* (1020), proposed that a thrower imparts an impetus to a projectile, which persists until external forces like air resistance dissipate it. He distinguished between ‘force’ and ‘inclination’ (called “mayl”), arguing that an object gains mayl when opposing its natural motion. Motion continues, he said, until the mayl is spent, and a projectile in a vacuum would keep moving unless acted upon—consistent with Newton’s first law.

In the 12th century, Jewish scholar Hibat Allah Abu'l-Barakat al-Baghdaadi stated that a falling body’s acceleration results from the continuous action of its natural inclination. According to Shlomo Pines, this was the earliest negation of Aristotle’s law that constant force produces uniform motion, and a vague anticipation of classical mechanics’ principle that continuous force produces acceleration.

Influenced by Ibn Sīnā and al-Baghdaadi, 14th-century French priest Jean Buridan developed the theory of impetus, which later evolved into modern ideas of inertia, velocity, acceleration, and momentum. This work was advanced in 14th-century England by the Oxford Calculators, including Thomas Bradwardine, who studied and formulated laws of falling bodies. The concept that a body’s main property is uniformly accelerated motion (as in falling) was worked out by these Oxford Calculators.

**Early Modern Age** Key figures in the early modern period are Galileo Galilei and Isaac Newton. Galileo’s final statement on mechanics, especially falling bodies, appears in his *Two New Sciences* (1638). Newton’s *Philosophiæ Naturalis Principia Mathematica* (1687) gave a detailed mathematical account of mechanics using calculus, forming the basis of Newtonian mechanics. There is debate over priority: Newton’s *Principia* is the seminal work, and many results required calculus, but ideas about inertia and falling bodies were developed earlier by scholars like Christiaan Huygens and medieval predecessors. Assigning precise credit is difficult because scientific language and proof standards have changed; medieval ideas may be seen as equivalent to modern statements or as preliminary hypotheses.

**Modern Age** Two major modern developments are Einstein’s general relativity and quantum mechanics, both emerging in the 20th century, partly from 19th-century ideas. Modern continuum mechanics—covering elasticity, plasticity, fluid dynamics, electrodynamics, and thermodynamics of deformable media—developed in the second half of the 20th century.

**Types of Mechanical Bodies** The term “body” covers a wide range of objects: particles, projectiles, spacecraft, stars, machine parts, solids, and fluids (gases and liquids). Sub-disciplines of mechanics also differ by the nature of the bodies studied. Particles have little internal structure and are treated as mathematical points in classical mechanics. Rigid bodies have size and shape but remain simple, close to the particle model.