Inertia

Inertia is the inherent tendency of a moving object to keep moving and a stationary object to stay still, unless a force alters its velocity. This principle is a cornerstone of classical physics, formalized by Isaac Newton in his first law of motion, which is also called the Principle of Inertia. The property is a key expression of mass, a fundamental quantitative attribute of physical systems. Newton stated: "Every object perseveres in its state of rest, or of uniform motion in a right line, except insofar as it is compelled to change that state by forces impressed thereon." In his 1687 work *Philosophiæ Naturalis Principia Mathematica*, he defined inertia as a property: "The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavours to persevere in its present state, whether it be of rest or of moving uniformly forward in a right line."

The concept has a long history. Professor John H. Lienhard notes that the Mozi, a Chinese text from the Warring States period (475–221 BCE), contains the earliest known description of inertia. Before the European Renaissance, Western philosophy followed Aristotle (384–322 BCE), who believed objects on Earth moved only as long as a force was applied, because gravity, friction, and air resistance often mask inertia by slowing moving objects to a stop. Aristotle explained projectiles as being kept in motion by the surrounding medium. This view was challenged over nearly two millennia. Lucretius, following Epicurus, argued that matter’s default state was motion, not rest. In the 6th century, John Philoponus criticized Aristotle’s inconsistency between projectiles and the void, proposing that motion was sustained by a property imparted to the object, not by the medium. Though still requiring a power to maintain motion, this was a key step. Philoponus’s ideas were opposed by Averroes and many scholastics but supported and developed by others in the Islamic world. In the 11th century, Ibn Sina (Avicenna) argued that a projectile in a vacuum would not stop without an external force.

In the 14th century, Jean Buridan rejected the idea that impetus dissipated spontaneously, claiming that a moving object would be stopped only by air resistance and its own weight. He believed impetus increased with speed, similar to the modern concept of momentum, but saw his theory as a modification of Aristotle’s, maintaining a fundamental difference between motion and rest. He also thought impetus could be circular, as in celestial bodies. Buridan’s student Albert of Saxony and the Oxford Calculators conducted experiments that further weakened the Aristotelian model, and Nicole Oresme pioneered graphing laws of motion. Shortly before Galileo, Giambattista Benedetti modified impetus theory to involve only linear motion, stating that any moving body has a natural tendency to move in a straight line, not a curved one, citing a rock in a sling as an example.

According to historian Charles Coulston Gillispie, inertia entered science as a physical consequence of Descartes’ geometrization of space-matter, combined with the immutability of God. Isaac Beeckman was the first physicist to fully break from Aristotle’s model in 1614. Johannes Kepler first introduced the term “inertia” in his *Epitome Astronomiae Copernicanae* (1617–1621), deriving it from the Latin for “idleness” or “laziness.” However, Kepler defined it only as resistance to movement, assuming rest was the natural state. It was not until Galileo and Newton unified rest and motion into one principle that the term took on its modern meaning.